US Immigration and Customs Enforcement (ICE) agents will be sent to airports to help with increasingly long lines at security checkpoints, the Trump administration has said.

President Donald Trump posted on social media that ICE agents will go to airports on Monday, while border tsar Tom Homan told CNN that his team was working out the details of the plan.

The union that represents the Transportation Security Administration (TSA) officers criticised the move, saying staff "deserve to be paid, not replaced by untrained, armed agents".

Travellers have faced hours-long airport security queues in recent days as the partial government shutdown affecting the Department of Homeland Security continues.

The DHS has gone without funding since mid-February, after Congress failed to reach an agreement - leaving normal airport security staff without pay.

Trump posted on Truth Social: "On Monday, ICE will be going to airports to help our wonderful TSA Agents who have stayed on the job."

Trump's comments came a day after he threatened to send ICE agents to help TSA agents at airports if Democrats did not immediately agree to fund DHS.

"I will move our brilliant and patriotic ICE Agents to the Airports where they will do Security like no one has ever seen before," Trump posted on Saturday.

The ICE agents will not be directly involved in screening passengers, Homan told CNN's State of the Union.

Instead, they will be used to free up TSA agents to conduct the screenings they are trained to do, he said.

Homan suggested ICE agents would cover entry and exit points so "that relieves the TSA officer to go to screening and to reduce those lines".

The border tsar added that he is still working with the TSA and ICE to iron out the details, including how many agents would be involved, but said the plan will be finalised before agents are deployed on Monday.

Speaking to ABC News' This Week, Transport Secretary Sean Duffy suggested a wider role for ICE.

He said ICE had the experience to help the TSA as they ran the "same type of security machines at the southern border" for packages and people.

"We have ICE agents who are trained and can provide assistance," he added.

In a statement to the BBC's News partner CBS News, a DHS spokesperson said the president "is using every tool available" to help American travellers.

"While the Democrats continue to put the safety, dependability, and ease of our air travel at risk, President Trump is taking action to deploy hundreds of ICE officers, that are currently funded by Congress, to airports being adversely impacted," the DHS spokesperson said on Sunday.

The BBC has contacted ICE for comment.

House Minority Leader Hakeem Jeffries responded to the announcement on Sunday, telling CNN the deployment of ICE agents is "the last thing that the American people need".

"These are untrained individuals when it comes to doing the current job that they have, for the most part - let alone deploying them in close exposure and highly sensitive situations at airports across the country," Jeffries said.

The stalemate over DHS funding has forced TSA agents to go without pay for more than a month, leading to a spike in agents absent from work. More than 400 agents have also quit their jobs since the partial shutdown began, the White House has said.

The TSA screens passengers and luggage for hazardous items.

"Our members at TSA have been showing up every day, without a paycheck, because they believe in the mission of keeping the flying public safe," said Everett Kelley, the president of the American Federation of Government Employees (AFGE), which represents of TSA workers.

"They deserve to be paid, not replaced by untrained, armed agents who have shown how dangerous they can be," Kelley said in a statement.

Democrats have demanded reforms to ICE after federal agents killed two Minneapolis residents, Alex Pretti and Renee Good, who were protesting against the Trump administration's immigration raids in Minnesota in January.

The Democrats have asked the DHS to bar immigration agents from wearing face masks, provide better identification for officers and tighten rules for obtaining warrants.

A bill to fund DHS and provide payments to airport TSA agents failed to advance in the Senate on Friday.