Secretary of Defense Mark Esper is asking European allies to "pick up that tab" on US military projects in their countries that are being defunded to pay for President Donald Trump's border wall.

"Look, if you're really concerned then maybe you should look to cover those projects for us," Esper said in London Thursday, explaining that some of the $3.6 billion that was diverted had been intended for infrastructure projects in these countries.

The Trump administration has been putting pressure on America's European allies to increase their contributions to the alliance.

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Secretary of Defense Mark Esper has reportedly asked European allies to "pick up that tab" on US military projects in their countries that are being defunded to pay for President Donald Trump's border wall.

For the longest time, the president assured the American people that Mexico would pay for the wall. Earlier this year, Trump backtracked on his promises. And now it appears that Europe is being tapped to cough up funds for the projects suddenly lost funds to pay for Trump's wall on the US-Mexico border.

The Pentagon revealed Wednesday that it is diverting $3.6 billion from 127 Department of Defense construction projects to pay for the wall. The funds are being transferred to the Army Corps of Engineers to construct 175 miles of border barriers.

Roughly 40 of the affected projects are in Europe, Defense News reported.

The move affects not only domestic projects, but also a number of overseas projects. Speaking to reporters in London, Esper said that European allies should cover the costs of some of these projects.

"The message that I've been carrying," Esper explained, "has been about the increase in burden sharing. So part of the message will be, 'Look, if you're really concerned then maybe you should look to cover those projects for us' because that's going to build infrastructure in many cases in their countries."

"Part of the message is burden sharing, 'Maybe pick up that tab,'" he added, according to Reuters.

The Trump administration has made extracting more money from America's European allies a priority, and the NATO alliance has, at times, taken a beating Trump and from members of his administration. Trump has called for US allies to pay more for bases in their countries, and for NATO members to increase their defense spending to 2% of their GDP, which NATO set as a goal by 2024; most of NATO's members still fall short of this.

Esper, who became secretary of defense in July, has embraced the need for fair burden sharing, adding more weight to the president's rhetoric on spending that has caused some concern within the alliance at a time when there are concerns about a resurgent Russia.

Before he met with NATO leaders in June, the Department of Defense said that the focus would be "reinforcing the US commitment to strengthening the NATO Alliance, ensuring more equitable burden sharing, bolstering NATO readiness and addressing regional security issues."

In the coming days, Esper will meet with several of his counterparts in Europe. Speaking to the press Thursday, he said that he will "continue to push our allies to raise their numbers," referring to spending on collective security.