Washington DC - Republicans sounded the alarm Thursday over potential turmoil at US airports as the government shutdown threatens to drag into November, warning of ruined travel plans for millions of Americans.

With the standoff in its fourth week, President Donald Trump's Republicans and the opposition Democrats are facing increasing pressure to end a stalemate that has crippled public services.

More than 60,000 air traffic controllers and Transportation Security Administration officers are working without pay, and the Trump administration joined Republicans in Congress to warn that increasing absenteeism could mean chaos at check-in lines.

"We are rounding into a holiday season, as we all know, and we're in the middle of the height of the football season. This is peak travel time for the US," House Speaker Mike Johnson told a news conference.

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"Hundreds of thousands of Americans are going to travel to football games this weekend, for example, and if the current trajectory continues, many Americans could miss watching their favorite teams and reconnecting with friends and family."

Airport workers calling in sick rather than working without pay – leading to significant delays – was a major factor in Trump bringing an end to the 2019 shutdown, the longest in history at 35 days.

In normal times, 5% of flight delays are the result of staffing shortages, but that has increased to more than 50%, Johnson said.

He told reporters that 19,000 flights were delayed from Saturday to Monday and that this rate was "only going to increase" as the shutdown continues.

Johnson said airport workers were finishing long shifts and then taking on second jobs as Uber drivers or delivering food.

"The longer the shutdown goes on, and as fewer air traffic controllers show up to work, the safety of the American people is thrown further into jeopardy," he warned.

Transportation Secretary Sean Duffy said he had spoken to staff in control towers who were voicing deepening anger over the shutdown – a standoff over healthcare funding that has led to 1.4 million federal workers going without pay.

"I want them coming to their facilities and controlling the airspace, but they're having to make decisions about how they spend their time, to make sure they put food on their table, feed their kids, and support their family members," he said.

With no end to the shutdown in sight, the gridlock is beginning to take a personal toll on lawmakers, who fly out of Washington most weekends to return to their home districts.

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During an earnings call with analysts on Thursday American Airlines CEO Robert Isom said Ronald Reagan Washington National Airport, five miles from the capital, had suffered "operating delays and issues with air traffic control."

After weeks of failed daily votes on a House-passed resolution to reopen the government, the Senate also rejected a bill Thursday to guarantee pay for troops and some federal employees who have been working for nothing.

Republicans had hoped that a blockade on troop pay would be seen by some Democrats as politically toxic and might be a catalyst to finally break the party's united stance on the shutdown.

All but three Democrats voted against the bill, however, arguing that it would have given Trump too much sway over who gets paid and who doesn't, while offering no help for 750,000 workers placed on enforced leave without pay.

Democrats say the only path to reopening the government is a Trump-led negotiation over their demands to extend subsidies that make health insurance affordable for millions of Americans – the key sticking point in the standoff.

But Trump has insisted he won't negotiate with Democrats until the shutdown is over.