US President Donald Trump told US government agencies to prepare to "immediately repel, repatriate, and remove" undocumented immigrants as part of a wider effort to fight what he's termed an "invasion" across the southern border.

Trump's order comes as US officials announce the deployment of additional active duty troops to the border, and as processes that allow swift deportations are expanded.

The US also moved to cancel all refugee travel and processing, leaving thousands stranded as they wait to come to the US.

White House Press Secretary Karoline Leavitt said Trump is "sending a very strong message" to people around the world with the actions.

An executive action, posted to the White House's website, argues that the measures are necessary to prevent undocumented immigrants from entering the US, "invading" US communities and "imposing billions of dollars of costs" on state and local governments.

Speaking to reporters after Trump signed the order, Ms Leavitt said that the border was a "number one priority" for the president.

"If you are thinking about breaking the laws of the United States of America, you will be returned home," Leavitt said of Trump's message. "You will be arrested. You will be prosecuted."

Ms Leavitt also confirmed that 1,500 additional troops would be sent to the US-Mexico border.

The newly inaugurated president has signed a flurry of immigration and border-related actions and decrees this week aimed at cracking down on immigration.

The orders include tackling the definition of birthright citizenship and declaring illegal immigration at the border a national emergency.

A notice posted on the website of the Federal Register says the expedited removal policy took effect on the evening of 21 January.

The policy, which has traditionally been limited to undocumented migrants detained within 100 miles (160km) of the country's international borders, now allows officers to use it anywhere in the US.

"The effect of this change will be to enhance national security and public safety - while reducing government costs - by facilitating prompting immigration determinations," the notice reads.

It adds that the change will allow the Department of Homeland Security to address "the large volume of aliens" in the US illegally and ensure the "prompt removal...of those not entitled to enter, remain, or be provided relief or protection".

The expanded policy could be challenged in court.

Until now, "unauthorised" immigrants detained in the US were given a notice to appear in immigration court, where they could present their case for asylum.

Deportation proceedings typically couldn't begin until a judge issued a decision.

But earlier this week, Trump cited an immigration law - 212(f) - that allows the president to suspend the entry of foreign nationals deemed "detrimental" to the US.

Citing internal documents and US officials, CBS has reported that the policy also applies to the US border with Canada and to Customs and Border Protection's maritime sectors, such as Florida.

The separate order to stop refugee travel and processing comes just days after Trump signed an executive order suspending the US's Refugee Admissions Program (USRAP), saying America "lacks the ability to absorb large numbers of migrants, and in particular, refugees, into its communities in a manner that does not compromise the availability of resources for Americans".

It halts USRAP until "further entry into the United States of refugees aligns with the interests of the United States", the order says.

More than 1,600 Afghans who had already been approved to come to the US have had their travel plans cancelled, prompting an advocacy group, Afghan USRAP Refugees, to pen an open letter to President Trump.

More than 3,000 other Afghan nationals are waiting in Albania to be resettled in the US.