Donald Trump says he won't protect young people from deportation unless the Democrats commit to pay for his border wall.

The President sent a tweet saying that he would never back a programme that allows people to be saved from being deported if they arrived in the US as children. The most high profile of his immigration measures is the wall across the Mexican border, but it also includes a range of visa programmes.

Mr Trump repeatedly promised during his campaign that he would build a new wall right across the US border with Mexico. But he has run into trouble getting the funding for it, with the work likely running up a substantial bill.

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Now he has said that unless that funding is found, he will not support the Deferred Action for Childhood Arrivals (DACA) programme. He brought the program to an end in September, but Democrats have repeatedly pushed for it to be reinstated so that young immigrants won't be deported.

The debate is likely to continue to be a pivotal issue in 2018 as the US heads towards midterm congressional elections in November.

Mr Trump has said that he will not make any changes to the DACA programme without full support from Democrats for an end to various visa programs and the construction of a wall along the southern U.S. border. Such a deal is unlikely to be palatable for Democrats, who see the DACA programme as one of the Obama presidency's biggest victories.

"The Democrats have been told, and fully understand, that there can be no DACA without the desperately needed WALL at the Southern Border and an END to the horrible Chain Migration & ridiculous Lottery System of Immigration etc," Mr Trump posted on Twitter on Friday.

Representatives for Senate Democratic Leader Chuck Schumer and House Democratic Leader Nancy Pelosi said they would not negotiate the issue in the media but looked forward to serious talks after lawmakers return to work in Washington early next month.

The Senate is set to resume its work on 3 January, while the U.S. House of Representatives restarts its session on 8 January.

Trump promised to build a border wall as a presidential candidate and has continued to press for it publicly.

He has also called for an additional "merit based" assessment for U.S. visa recipients.