- The government has been in partial shutdown for 20 days, leaving about 800,000 federal employees without pay.
- Budget talks have come to a standstill as Democrats - who control the House of Representatives - refuse to give him the money.
- Trump has threatened again to declare a national emergency but the move would almost certainly face a legal challenge
- He made the border wall a key campaign promise - and pledged to make Mexico pay for it.
US President Donald Trump has threatened again to declare a national emergency to fund a border wall without Congress's approval.
"I have the absolute right to declare a national emergency," he told reporters as he headed to an event at the border.
He also said Mexico would "indirectly" pay for the wall - contradicting an earlier campaign memo.
The government has been in partial shutdown for 20 days, leaving about 800,000 federal employees without pay.
President Trump refuses to sign legislation to fund and reopen the government if it does not include $5.7 billion (£4.5 billion) for a physical barrier along the US-Mexico border.
But budget talks have come to a standstill as Democrats - who control the House of Representatives - refuse to give him the money.
Republican leaders insist the party stands behind the president, although some Republican lawmakers have spoken out in favour of ending the shutdown.
On Thursday, Mr Trump visited a border patrol station in McAllen, in the Rio Grande Valley of Texas.
He said that if Congress did not approve funding for the wall, he would "probably... I would almost say definitely" declare a national emergency to bypass lawmakers.
Analysts say that while presidents can direct military construction projects during war or national emergencies, the move would almost certainly face a legal challenge, and be accused of violating constitutional procedures.
The money would also have to come from funds allocated by Congress for other purposes - which some Republicans would also oppose.
One of his supporters though, Republican Senator Lindsey Graham, said it was "time for President Trump to use emergency powers to fund the construction of a border wall/barrier".
Democratic Senator Joe Manchin said that while a national emergency declaration by Mr Trump would be "wrong", it may be the president's "only way out" of the deadlock.
Analysts say such a move would provide political cover to reopen government while allowing Mr Trump to argue he had done all he could to fulfil one of his main campaign promises.
Mr Trump has been briefed on one plan that would involve diverting funding allocated to reconstruction projects in disaster areas, including Puerto Rico, to pay for the wall, US media report.