WASHINGTON — NATO Secretary General Jens Stoltenberg is due to be invited by Congress in April, sources familiar with the planning said Monday.

The invitation comes as NATO celebrates its 70th anniversary in April, but also comes at a tense time in the alliance. According to multiple reports, President Donald Trump is mulling a demand that nations that host U.S. troops pay their costs, plus 50 percent.

House Speaker Nancy Pelosi, D-Calif., is inviting Stoltenberg, a source said. She consulted with Senate Majority Leader Mitch McConnell, R-Ky., Sen. Minority Leader Chuck Schumer, D-N.Y., and House Minority Leader Kevin McCarthy, D-Calif., and all of them support her forthcoming invitation.

McConnell is joining in the invitation, according to McConnell spokesman David Popp. “Leader McConnell and Speaker Pelosi have agreed to invite Secretary General Jens Stoltenberg to address a joint session of Congress this spring. We will have additional details of the secretary general’s speech in the weeks to come.”

Pelosi and a delegation of U.S. lawmakers visited Brussels in February, where they met with NATO leadership. There, she sought to reassure allies worried about Trump’s rhetoric that Congress is a co-equal branch of government, and it supports NATO.