WASHINGTON, United States (AP) — President-elect Donald Trump on Friday tweeted that Mexico will reimburse American taxpayers for a new border wall and that US money spent will be for the “sake of speed”.

His tweet came as top aides consider a plan to ask Congress to ensure money is available in US coffers for the wall, but to rely on existing law that already authorises fencing and other technology along the southern border.

The potential approach was confirmed Thursday by two congressional officials and a senior transition official with knowledge of the discussions; all spoke on condition of anonymity because they were not authorised to discuss the matter publicly.

Trump said in a tweet early Friday: “The dishonest media does not report that any money spent on building the Great Wall (for sake of speed), will be paid back by Mexico later!”

During his campaign, Trump repeatedly told voters if elected he would build a wall along the US southern border and make Mexico pay for it.

Trump transition spokesman Sean Spicer said putting US money upfront “doesn’t mean he’s broken his promise”. In an interview Friday on ABC’s “Good Morning America”, Spicer said: “I think he’s going to continue to talk to them (the Mexican Government) about that.”

The approach could also stave off a legislative fight that Trump might lose if he tried to get Congress to pass a measure authorising the kind of border wall he promised during the campaign.

It’s not clear how much could be done along the 2,000-mile border without additional actions by Congress. Lawmakers passed the Secure Fence Act of 2006, but most of those 700 miles have already been built. Some areas are in much better shape than others, though, and long stretches are made up of fencing that stops vehicles but not pedestrians.

But whatever steps might be taken without Congress’ approval would be likely to fall short of the extravagant new wall on the border that Trump repeatedly said Mexico would pay for during his campaign for the White House. And despite Congress’ involvement in approving any spending, such an approach might also open Trump to charges of circumventing the House and the Senate to take unilateral actions, something he repeatedly criticised President Barack Obama for doing. A spending bill including money for border construction could also provoke a legislative showdown given potential opposition from Senate Democrats.

Still, several lawmakers and congressional officials said the administration could have significant flexibility in taking additional steps without Congress’ approval.

“There’s a lot of things that can be done within current law,” said Republican Mario Diaz-Balart, a longtime proponent of comprehensive immigration reform, though he emphasised that a lasting solution on immigration would take action by Congress. “You cannot minimise the potential impact of the administration doing what they can do under the law,” he said.

However, some immigration hard-liners have already expressed the desire to see Congress take a vote, given how prominent the wall was during Trump’s presidential campaign, and their desire to act on the issue.

Trump’s vow to build an impenetrable, concrete wall along the southern border was his signature campaign proposal. “Build the wall!” supporters would chant at his rallies. “Who’s going to pay for it?” Trump would ask them. “Mexico!” Trump often promised the wall would be built of hardened concrete, rebar and steel as tall as his venues’ ceilings, and would feature a “big, beautiful door” to allow legal immigrants to enter.

Most experts viewed such promises as unrealistic and impractical, and Trump himself sometimes allowed that the wall would not need to span the entire length of the border, thanks to natural barriers like rivers. After winning the election, he said he’d be open to stretches of fencing.