The US government reopened on Monday with about 800,000 federal workers returning after a 35-day shutdown triggered by President Donald Trump's demand for a border wall as politicians geared up for talks to avoid another standoff in three weeks.

The longest shutdown in US history ended on Friday when Trump and Congress agreed to temporary government funding - without money for his wall - as effects of the shutdown intensified across the country.

Throughout the five-week shutdown, the US economy lost about $11bn during the five-week period, the nonpartisan Congressional Budget Office (CBO) said.

But CBO expects eight billion dollars to be recovered as the government reopens and federal workers receive back pay.

The CBO said the cost of the shutdown will make the US economy 0.02 percent smaller than expected in 2019. But researchers said more significant effects will be felt by individual businesses and workers, particularly those who went without pay.

Trump had demanded $5.7bn in funding to help build his long-promised wall along the US-Mexico border, which he says is necessary to stop irregular immigration, human trafficking and drug smuggling.

It remained to be seen how politicians planned to address border security before the February 15 deadline to avoid another closure.

A committee of politicians from both parties will try to negotiate a compromise on border security as Congress prepared to reconvene on Monday.

"There will have to be compromise," US Representative Dan Kildee, a member of the House Democratic leadership, told CNN in an interview on Monday.

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He urged fellow politicians to ignore Trump's wall rhetoric and "put our heads down and do our job".

Trump has vowed to shut down the government again unless an acceptable border deal is reached. On Sunday, he expressed scepticism that such an agreement could be made.

He also left open the possibility of declaring a national emergency to get money for the wall, an extraordinary move that Democrats and some Republicans have vowed to fight and that would likely face a court challenge.

More technology, more border agents

Trump, who had initially said Mexico would finance the wall, did not address the government reopening or shutdown talks in a series of tweets on Monday morning.

Democrats, eager to capitalise on their political victory, oppose any money for a wall, but say they back additional funding for security measures along the border, such as increased technology and more Border Patrol agents.

Republicans, who controlled both the Senate and the House when the shutdown began December 22, are wary of a repeat closure. Polls show the public mostly blamed Trump and his party for the standoff.

Any agreement must win over House Speaker Nancy Pelosi and her fellow Democrats, who now control the House as well as enough Democrats to reach the 60-vote threshold for funding bills in the Republican-controlled Senate.

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Democratic Senator Maggie Hassan told CNN she was open to a deal with money for strategic border fencing but not Trump's wall.

Federal workers are expected to get paid this week for the five weeks of missed paycheques.

Federal contractors and businesses that relied on federal workers' business, however, face huge losses, although some politicians are pushing legislation to pay contractors back as well.

House Democrats plan to offer legislation to increase civil servants' pay, Representative Don Beyer, whose Virginia district is home to many federal workers, tweeted on Monday.

Some politicians are also examining ways to outlaw any future shutdowns, the New York Times reported over the weekend.