Travellers across the US are facing unusually long airport lines, with some of the worst delays reported in Houston, where security wait times have stretched beyond four hours amid a partial government shutdown.

Hundreds of Transportation Security Administration (TSA) agents continue to miss paychecks and call out of work, leaving large gaps in airports' abilities to screen passengers.

But nearly 40% of the security staff at the George Bush Intercontinental Airport in Houston did not show up for work - the highest such rate in the country, US media report.

Officials fear that conditions "will only get worse at airports across the US until Congress ends this shutdown".

Houston's airport is operating just one-third to 50% of its TSA checkpoints, said Jim Szczesniak, director of aviation for the Houston Airport System.

"That's 100% spring break loads going through the airport, being processed through less than 50% of our TSA lanes," he noted, referring to the spring break travel season. "That is not sustainable."

A flurry of sporting events in Houston this weekend may bring more passengers to the airport, further clogging lines. The Astros' season opener is scheduled for this weekend, along with a golf open and a pair of NCAA Tournament Sweet 16 matchups.

Ahead of that, the TSA plans to deploy at least two dozen officers from its National Deployment Office to the Bush airport on Thursday, the Houston mayor's office told media partner CBS News.

On Monday, the Trump administration said hundreds of US Immigration and Customs Enforcement (ICE) agents had been sent to 14 airports in cities including New York, Atlanta and Houston to help fill the void.

US President Donald Trump touted the success of the move on Wednesday, saying they're doing "an unbelievable job" at airports. He indicated he would also send the National Guard "if we need to" in order to assist TSA and ICE. He blamed Democrats for the worker shortages, saying they "don't want to pay" the agents.

Democrats in Congress have refused to fund the Department of Homeland Security without new limits on immigration agents, following public outrage over the shooting death of the two US citizens in Minneapolis.

Republicans have rejected Democratic ​proposals to fund TSA amid ongoing negotiations over ICE reforms.

There is little sign that the funding standoff is close to being resolved.

Billionaire Elon Musk offered to fund pay for TSA workers during the partial shutdown but the White House rejected the offer on Wednesday, several sources told CBS News.

A top TSA official said on Wednesday that more than 450 TSA workers have quit since the partial shutdown began.

While that's just a fraction of some 50,000 TSA agents who screen passengers, the resignations, paired with thousands of absences, are creating longer-than-usual queues.

During testimony before a congressional oversight committee, the TSA's acting chief, Ha Nguyen McNeill, warned on Wednesday that the turnover will have "dire" consequences as the US prepares to co-host the FIFA World Cup this summer.

"At this point, if we bring on any new [TSA agents], those folks will not be deployed in time by FIFA," she said.

The TSA is monitoring the staff attrition rates closely, McNeill said, adding: "If we see any spikes, we'll have to pivot and assess how we are going to staff the FIFA locations adequately."