With little time remaining to craft a deal over funding security operations on the US-Mexico border, a bipartisan group of politicians was to meet in a public work session on Wednesday even as President Donald Trump maintained a hard line on constructing a massive wall.

Congressional negotiators are up against a February 15 deadline for agreeing on funding through September 30 for several federal agencies, including the Department of Homeland Security and its border operations.

Realistically, Republican and Democratic legislators have little more than a week to settle differences and still give the full House of Representatives and Senate time to debate and vote on any deal.

A 35-day partial shutdown was triggered on December 22 when Trump refused to sign any funding bills that did not contain at least $5.7bn for a wall along the southwestern US border.

Throughout the shutdown - the longest of its type in US history - polls consistently found that most Americans blamed Trump.

Faced with steadfast opposition in the Democratic-majority House, Trump relented on Friday, agreeing to re-open federal agencies temporarily even though he did not get his $5.7bn request. In return, Congress agreed to form a special panel to negotiate a border security deal.

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Trump has threatened a resumption of the record-long shutdown if the panel fails to find common ground or produces a plan he does not like.

In a tweet on Wednesday, Trump warned: "If the committee of Republicans and Democrats now meeting on Border Security is not discussing or contemplating a Wall or Physical Barrier, they are Wasting their time!"

Physical barriers have long been installed on parts of the border to keep out illegal drugs and undocumented immigrants and more are under construction.

It was unclear whether Trump, who views the current arrangement as insufficient, would accept a simple continuation of such installations.

Negotiations over border

Building a wall on the US's southern border - with Mexico paying for it - was one of Trump's most oft-repeated promises during the 2016 presidential campaign. Mexico has refused to pay for a wall.

Democrats, arguing that a border wall is ineffective, say they want a mix of security tools such as drones, sensors, scanning devices and fences, along with more border patrol agents.

Wednesday's committee meeting might be the only public session since behind-the-scenes negotiations are the stage for the real bargaining.

The session is expected to mainly allow the seven Senate negotiators and 10 House negotiators an opportunity to make opening statements.

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The committee is headed by House Appropriations Committee Chairwoman Nita Lowey, a Democrat, and Republican Richard Shelby, who is chairman of the Senate Appropriations Committee.

With a mix of wall supporters and opponents, it is unclear whether the panel will be able to reach any agreements.

If Congress does not grant his request, Trump has threatened to declare a "national emergency" in order to take existing funds appropriated by Congress for other purposes - possibly from the Department of Defense, for example - to build his wall.

There is bipartisan opposition in Congress to that plan, which likely would spark legal challenges since the US Constitution gives Congress the power to appropriate funds and direct their use.