President Donald Trump’s promise to use existing funds to begin immediate construction of a wall on the US-Mexico border has hit a financial obstcle, according to a document seen by Reuters.

The rapid start of construction, promised throughout Trump's campaign and in an executive order issued in January on border security, was to be financed with “existing funds and resources” of the Department of Homeland Security (DHS). But so far, the DHS has identified only $20 million that can be re-directed to build up the wall, according to a document prepared by the agency and distributed to congressional budget staff last week.

The lack of existing funds at the DHS means that Trump will need to convince Congress to include additional funding for the border wall in the 2018 federal budget. According to the latest DHS estimates, building the wall could cost $21.6 billion, and several Republican lawmakers have already said they will not support such an expense unless it is offset by public spending cuts, reported El País.

Trump had also claimed that Mexico would be paying for the wall. Later he reviewed this statement, and said that the US would pay for it first, and Mexico would reimburse US taxpayers at a later date.

The White House has been gradually lowering its demands, and its latest claim was that the wall would be built using existing DHS funds.