Russia has threatened to veto a UN Security Council resolution reauthorizing cross-border deliveries of vital aid into opposition-held north-western Syria. The closure of the last route from Turkey would immediately put more than 3 million people at risk of starvation. The BBC's Anna Foster has been following one of the last UN aid convoys that could be allowed into the country.

Umm Ali carefully boils a pot of water to cook dinner for her seven children. She feeds the fire with scraps of cardboard and rubbish, whatever she can find to keep the flames going. The meal she makes is small and basic, because here at the Al-Sadaqah camp, in the Idlib countryside, supplies are scarce. She's thankful for the food aid she receives, but even that isn't enough.

"Every day the kids go to the landfill to collect aluminium cans, nylon bags and iron. They sell them for a pittance, it's just enough to buy four packets of bread. That's only enough for one meal, our breakfast," she says.

The UN says the number of Syrians in need is higher now than at any point during the 11-year civil war, as the toxic mix of years of fighting, the Covid-19 pandemic and the war in Ukraine have devastated the country's economy. The World Food Programme (WFP) says the cost of food has risen by 800% in just two years.

At a Turkish compound just a few kilometres from the border, lorries are loaded with humanitarian aid. It could be anything from food to temporary shelters, or vital medicines.

Hakim Froissart is logistics officer for the WFP in southern Turkey, one of the key people who keeps things running smoothly.

"We're waiting for the vote, we don't know what will happen," he says.

"If there is no resolution we don't have any mandate to operate any more. So we would have to stop the operations. And we would have to find a solution. But there's no-one who can take over," he adds.

Part of the reason for that is the sheer logistical effort needed here. Around 1,000 loaded lorries make the journey across the border every month. In the last few weeks there have been more than usual. Aid agencies have been so concerned about the potential of the crossing being closed that they have been sending extra shipments to build up a small surplus on the other side.

The process is well practised and fast. Two rows of lorries line up back to back. One is Turkish, the other Syrian. They pull together until they're touching, and piles of cardboard boxes are pushed from one into the other, ready to be transported to the border and beyond.

The whole cross-border operation only exists because of a UN mandate which allows it to happen without the permission of Syrian President Bashar al-Assad, Russia's ally.

The north-west of the country is outside his control, in the hands of the jihadist alliance Hayat Tahrir al-Sham and Turkish-backed rebel groups. The Syrian regime has only facilitated a small amount of what is called "cross-line" aid. That means it crosses the front lines within a country, rather than international borders.

WFP food aid that has come via Damascus has fed less than 50,000 people. From the hub at the Turkish crossing, the UN and its partners sustain 1.4 million. Despite that, Russia believes President Assad's sovereignty takes precedence, and that the job of providing aid should lie in Syrian hands.

Bab al-Hawa is the only aid route left. There used to be others, allowing lorries to arrive in Syria from Iraq and Jordan. But Russia already used its veto to close them down. As a permanent member of the UN Security Council - along with the UK, US, France and China - it can block any mandate it chooses. Just a few months ago, it rejected a resolution that condemned Russia's invasion of Ukraine and called for a withdrawal of troops.

Exactly a year ago, there were again fears that the Bab al-Hawa crossing would be closed. But this time around the story is not only about Syria.

The vote in New York comes just days before US President Joe Biden's headline visit to the Middle East. It risks highlighting starkly how much influence the US has lost to Russia in the region.

In 2021, last-minute, high-level diplomacy between Mr Biden and Russian President Vladimir Putin kept the aid route open. But a lot has happened in the last 12 months. US-Russia relations are at their lowest ebb for decades, fractured further by the war in Ukraine, and it will be far harder than last year to crack those vital negotiations.

Alyssa is an inspector at the UN hub on the Turkish side of the border, and as the lorries are prepared she carefully slices through the tape holding one of the food boxes closed. Everything has to be checked, to make sure it's purely for humanitarian purposes.

She unpacks bags of bulgur wheat, lentils and chickpeas, rolling them in her hands to check there are no cuts or signs of re-sealing. Once the precious cargo is authorised the lorries are sealed and weighed, and sent on their way.

At the Bab al-Hawa crossing they pass straight through, without being stopped like most other vehicles. When they drive under the golden dome-topped arches of Gate Zero, they are on Syrian soil. Once they arrive at their destination aid agencies unpack the food and distribute it to displaced families.

This is a long conflict, and many people here have been living in makeshift tents for more than a decade. Children were born here and are being raised here. The tattered canvas tents of the displacement camps are the only home they have ever known.

Janne Suvanto, the WFP's Emergency Co-ordinator in north-west Syria, says an end to the last cross-border aid transfers would be devastating.

"I think it's a personal tragedy," he tells me.

"The good thing is that we've prepared a three-month buffer of food supplies. But people have used their coping mechanisms already. We know people are cutting their rations and quality of food.

"I think we can quite confidently say that there would be more preventable deaths if this border crossing is closed. Irrespective of any political viewpoints, it's important that we have humanitarian principles at front of mind."

Umm Ali's cramped tent in Idlib is a long way from New York, where the Security Council vote will take place. Her life, a daily battle to survive and protect her children, is far removed from quiet conversations and hushed diplomacy in UN corridors.

But decisions made thousands of kilometres away by some of the world's most powerful men and women have a direct and sizeable impact on her life.

"Of course we consider it something important and crucial, because we don't have money or work to secure food here," she says.

Umm Ali hopes diplomats in New York will remember her when they are making their choices.