US congressional negotiators said on Monday they reached a tentative deal on border security funding to avert another partial government shutdown due to start on Saturday, but an aide said it did not include the US$5.7 billion President Donald Trump wants for a border wall.

"We reached an agreement in principle" on funding border security programs through Sept 30, Republican Senator Richard Shelby told reporters.

"Our staffs are going to be working feverishly to put all the particulars together,” Shelby said.

He did not say whether Trump would get any money for the US-Mexico border wall.

It was not clear if Trump would embrace the agreement. His December demand for US$5.7 billion to help pay for the wall - rejected by congressional Democrats - triggered a 35-day partial government shutdown that ended last month without him getting wall funding.

A congressional aide, who asked not to be identified, said the outline of the deal included $1.37 billion for erecting new fencing along the southern border. That is about the same amount Congress allocated over the past couple years and far below what Trump has demanded.

The aide said none of the money would be for a "wall," which Trump has been touting since he launched his campaign for president in 2016. Democrats say the wall would be costly and ineffective.

Trump was holding a rally in the border city of El Paso, Texas, on Monday night to argue for the wall he says can protect Americans from violent criminals, drugs and a "tremendous onslaught" of migrant caravans.

Trump said he heard about the progress in the talks just before he took the stage in El Paso, but he did not discuss details.

"Just so you know - we're building the wall anyway," he said. "Maybe progress has been made - maybe not."

Detention beds

Under Monday's agreement, which must be fleshed out by congressional staff experts, Democrats would give up on a demand they floated on Friday night to cap the number of immigrant detention beds in the interior of the United States.

Democrats had complained that the Trump administration was increasing detention capacity as a way of speeding up deportations of illegal immigrants, some of whom were seeking asylum under US law.

But an overall cap - on borders and in the interior - would remain at 40,520 beds. The aide said that despite that cap, the number had actually grown to 49,057 and that under the deal, it would be brought down to the legal cap.

Democratic Representative Nita Lowey said on Monday night: "I hope by Wednesday we'll have a finished product." Lowey said she had been in touch with House of Representatives Speaker Nancy Pelosi, who she said "has confidence I have made the right decision."

Trump agreed to reopen the government last month for three weeks to allow congressional negotiators time to find a compromise on government funding for the rest of the fiscal year, which ends on Sept 30, to avert another shutdown.

In Washington, the small group of lawmakers leading the negotiations met for about two hours on Monday.

They said they wanted to seal a plan by Monday night to allow time for the legislation to pass the House and Senate and get Trump's signature by Friday, when funding is due to expire for the Department of Homeland Security, the Justice Department and several other federal agencies.

- Reuters