It was early Saturday, when hundreds of staff who operate a sprawling humanitarian operation at the Al-Hol displacement camp in northeast Syria were given a clear message: "Stop work."

The despatch was as abrupt as it was distressing for those who knew the daily work of stabilising the site, which holds 40,000 people, mostly women and children, displaced from areas previously controlled by the Islamic State group.

Water, sanitation and security were all upended at the huge camp, said a senior humanitarian worker familiar with its operation. Another facility in Syria's north-east, Al Roj, was also hit by the sudden order. IS suspects are held near both sites.

"All of a sudden, you [risked] real instability and violence rising, as well as, obviously, former ISIS on the street," said Susan Reichle, a retired USAID Foreign Service officer.

The dramatic stop-work order came after President Trump froze all foreign assistance provided by the United States, by far the world's biggest aid donor, on his first day back in office, calling for a review to ensure it abided by his "America First" foreign policy.

For days, aid officials and global charities had waited to understand the implications of that order. On Friday night, its scale became clear.

A leaked memo revealed that Secretary of State Marco Rubio was placing a 90-day halt on all existing foreign assistance - with the only exceptions for emergency food aid, and for military funding for Israel and Egypt.

As news of the freeze filtered through the ranks of the international aid community, stop-work notices began to arrive.

Programmes ranging from water sanitation projects to vaccination initiatives were thrown into chaos as contractors tried to understand the implications of the directive. BRAC, the world's largest non-profit, told the BBC that 3.5 million people would be affected by programmes it had suspended in four countries.

It felt "like an earthquake across the aid sector, with life-saving programmes in ruins", one veteran international aid worker told the BBC.

Those who support the freeze of US aid programmes, worth around $70bn per year, say they are vastly bloated, with Washington carrying too much of the weight compared to other Western nations. And they argue the government sends far too much money abroad that would be better spent on Americans at home.

The administration has made clear that it specifically opposes any projects supporting diversity and inclusion, transgender rights, family planning, abortion access and other issues - some of which have been long-targeted by Republican administrations. The freeze, they say, is designed to create an opportunity to root out wasteful spending.

"Every dollar we spend, every program we fund, and every policy we pursue must be justified with the answer to three simple questions," Mr Rubio has said. "Does it make America safer? Does it make America stronger? Does it make America more prosperous?"

The programmes affected, however, have been vast, triggering widespread shock and criticism in many parts of a global system intertwined with US funding. Aid contractors fearful of losing further funding have mostly been voicing these concerns privately, though some have spoken out.

On Monday evening, staff who work on the US programme countering the global spread of HIV could no longer log into their computer systems, according to Dr. Atul Gawande, former Assistant Administrator for Global Health at USAID, and an expert on the project.

Then-President Bush launched the President's Emergency Plan for AIDS Relief (Pepfar) in 2003. It now employs more than 250,000 doctors, nurses and other staff across 55 countries distributing anti-viral medication and doing critical preventative work. It is credited with saving millions of lives and suppressing the spread of HIV and Aids.

"The program is shuttered…. Services are shut," Dr Gawande told the BBC on Tuesday, saying clinics that served 20 million people with HIV were affected.

Paul Jordan, who works at the European Institute of Peace on repatriating foreign citizens from Al-Hol and Al-Roj, said much of his work funded by Washington had stopped immediately.

"In terms of immediate impact I've never seen anything as significant as this before," he told a UK parliamentary committee on Tuesday, adding the camps were set to be "in limbo" for months while the review was carried out.

"What that led to was in the last few days basically nothing being delivered within the camps," he said. "There was no camp administration, very little security, food wasn't delivered."

Later on Tuesday, as aid organisations clamoured for exemptions from the US government to continue programmes, the first signs emerged that the State Department was trying to limit the impact of its sweeping freeze.

The definition of "life-saving humanitarian assistance" allowed to continue was broadened beyond emergency food aid to include "core life-saving medicine", medical services, food, shelter and other provisions.

That guidance has reportedly seen Pepfar programmes restart, but whether preventative drugs - rather than just HIV treatments - are covered remains unclear.

Dr Gawande, who was appointed to a senior role in USAID under the Biden administration, said other programmes remained up in the air - including work combatting an Mpox outbreak in West Africa, bird flu monitoring across dozens of countries and initiatives targeting fentanyl trafficking.

"It was immediate and my immediate reaction was, this is catastrophic," he said of the effects of the freeze.

Asked about those specific programmes,, a State Department spokesperson said: "We are judiciously reviewing all the waivers submitted. The Secretary of State has the ultimate responsibility…to protect America's investments."

Blumont, the US contractor that coordinates aid work at Al-Hol and Al-Roj, said it received a waiver from the State Department late Monday allowing it to continue "critical activities" at the camps for two more weeks. However, it has no certainty beyond that time.

Asked by the BBC about the situation at the Syria camps and other projects, the State Department said "critical national security waivers have been granted," but didn't specify whether any related to Syria.

The new State Department guidance also said: "This waiver does not apply to activities that involve abortions, family planning conferences, administrative costs… gender or DEI (diversity, equity, and inclusion) ideology programs, transgender surgeries, or other non-life saving assistance."

Back in Washington, USAID headquarters have been roiled by the aid freeze.

Staff had been warned not to try and circumvent the directives and given strict orders not to communicate about the freeze outside the agency. An internal memo sternly warned that any breach would "result in disciplinary action".

Unnamed staff have since been accused of trying to "circumvent" the president's executive order and dozens of senior officials have been placed on administrative leave.

In the halls of the agency, picture frames that once displayed images of on-the-ground projects are now empty.

The world of US foreign assistance had been upended in a matter of days, said Dr Joia Mukherjee, an infectious diseases doctor from Harvard Medical School and the charity Partners In Health who helped advise on the creation of Pepfar.

"It's taking 20 years of goodwill and turning it into an instrument of terror, when people feel like if they touch the drugs, if they see a patient, they might get fired," she said.

As criticism mounted, on Wednesday, the State Department said the 90-day pause and review of foreign aid was "already paying dividends" to the US and its people.

"We are rooting out waste. We are blocking woke programs. And we are exposing activities that run contrary to our national interests. None of this would be possible if these programs remained on autopilot," it said.

Explaining why it was necessary to order a temporary suspension for all projects, rather than a more targeted approach, the State Department said: "It is impossible to evaluate programs on autopilot because the participants – both inside and outside of government – have little to no incentive to share programmatic-level details so long as the dollars continue to flow."

It added: "A temporary pause, with commonsense waivers for truly life-threatening situations, is the only way to scrutinize and prevent waste."

Thousands of kilometres away, in the Ugandan town of Masindi, Teddy Ruge is still grappling with the fall-out. He was told to "stop work" on Monday night, and the waivers given so far don't seem to allow him to restart his US government-funded farming project.

Mr Ruge employs small-plot farmers who grow a nutrient rich leaf called Moringa. The plant is sold to North America and Europe where it is used to fortify bread and other foods.

His farmers rely on a wage of around $70 per month, their incomes bolsted by a yearly grant of around $250,000 from USAID.

But that lifeline appeared to fall away, exactly a week after Mr Trump's executive order.

"We were actually preparing to have a meeting with all the farmers to talk about the new season and what to plant - a planning meeting," Mr Ruge told the BBC. "But now it's more of a funeral,"

He still doesn't know whether he is allowed to continue employing the farmers or if they can show up to work.

"From what I'm reading, our program is at jeopardy of being permanently canceled because it's at the edge of climate resilience and green manufacturing - which are not exactly at the top of the list of Trump's priorities," said Mr Ruge.

"It's really disheartening. So I have a few sleepless nights ahead of me."