Up to a third of the $6.7bn (£5.2bn) in Pentagon funds Donald Trump has identified to spend on a US-Mexico border wall has already been spent, officials have revealed.

During his emergency powers declaration last month, the US president announced he would divert billions of dollars from other Department of Defense projects towards the wall, in order to circumvent Congress.

It included $3.6bn (£2.8bn) in unspent military construction money, as well as $2.5bn (£1.9bn) in counterdrug funds and $600m (£462m) from an asset forfeiture account – the latter two not dependent on the emergency delaration.

But a spokesperson for the House appropriations committee told the Congressional Quartely (CQ) magazine that only $85m (£65m) remains in the counterdrug account, wiping out more than $2.4bn Mr Trump intended for the wall.

In order to recoup that money, the Pentagon is planning to redirect money from other programmes in order to bring the counterdrug account back up to $2.5bn, so that Mr Trump can then move it again into border wall funding.

“The Department would need to reprogramme additional funding into the account to reach the up to $2.5 billion that may be required for border security support,” Defense Department spokesman Christopher Sherwood told CQ.

Any repurposing of funds by the Pentagon, however, would require authorisation from the leaders of the four congressional committees which oversee the department.

Peter Viscolsky, Democratic chair of the House defense appropriations subcommittee, said in a statement he would refuse any request to repurpose Pentagon money for a wall.

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“I am adamantly opposed to the use of any funds provided by Congress to the Department of Defense for the unauthorized construction of a wall on the Southwest border,” he said.

“I and the other members of the House appropriations committee will carefully examine each element of the president’s proposal and the serious jurisdictional and constitutional concerns that it raises.”

The White House earlier this week announced it was moving forward with its wall plans, though in the face of lawsuits challenging the president’s declaration, officials told CNN they planned to only use funds from the counterdrugs programme and the Treasury Department’s asset forfeiture fund.