President Trump is privately pushing for the Pentagon to pay for his proposed border wall between the U.S. and Mexico, according to a new report.

The Washington Post reports that Trump made the suggestion to several advisers last week, saying it was a "national security" issue that the wall be funded.

Trump reportedly told Speaker Paul Ryan (R-Wis.) in a meeting last week that the military should pay for the wall. Ryan offered little reaction to the idea while senior Capitol Hill officials called its prospects unlikely, the newspaper reported.

The president has sought $25 billion in funding for his long-proposed wall, but a massive funding bill passed by Congress last week only appropriated $1.6 billion for border security, with funding provided for fencing and levees.

In a separate meeting with senior aides last week, the president reportedly voiced confidence that the Pentagon would have enough funds to cover the wall's cost under the omnibus spending bill.

Trump expressed frustration while signing the $1.3 trillion omnibus spending package, originally threatening to veto the bill because of its lack of funding for his border wall. The bill allots about $700 billion for the military.

Two advisers also told the Post that when Trump tweeted "Build WALL through M!" earlier this week, the "M" was intended to stand for "military." Trump originally said that Mexico would pay for the wall.

A senior Pentagon official told the newspaper that any reallocation of 2018 military funding for the border wall would require an act of Congress.

A budget amendment to find funds for the border wall in the fiscal 2019 budget would require 60 votes in the Senate, which the White House would be unlikely to find. Republicans currently hold a slim 51-49 seat majority.

Democrats on the House Armed Services Committee last fall added language to the 2017 National Defense Authorization Act (NDAA) that would have prevented any Pentagon funding from being used to cover the wall's costs.

That amendment, introduced by Rep. Beto O'Rourke (D-Texas), who is running against Sen. Ted Cruz (R-Texas) for Senate, was stripped from the final NDAA bill.

- John Bowden contributed

Updated: 1:40 p.m.