President Trump on Tuesday signed the treaty allowing Montenegro to join NATO, pledging that the country’s inclusion will not increase the United States’ payments toward the alliance.

“The United States will work to further strengthen our already strong relationship with Montenegro and looks forward to formally welcoming the country as the twenty-ninth member of the NATO Alliance,” the White House said in a statement. “President Trump congratulates the Montenegrin people for their resilience and their demonstrated commitment to NATO’s democratic values.”

The move comes amid rising tensions with Russia, which has long opposed NATO expanding its membership in Eastern Europe.

In a separate letter to the Senate also sent Tuesday, Trump asserted the inclusion of Montenegro into NATO “will not have the effect of increasing the overall percentage share of the United States in the common budgets of NATO” and “does not detract from the ability of the United States to meet or to fund its military requirements outside the North Atlantic area.”

NATO countries have pledged to spend 2 percent of their gross domestic product on defense. Only five, including the United States, currently meet that goal, and Trump during his presidential campaign called for member nations to increase defense spending to support the organization.

The White House added that Trump looks forward to a May 25 NATO leaders meeting in Brussels “and the opportunity to reaffirm those fundamental and enduring transatlantic values.”

“Montenegro will be there as well, signaling to other NATO aspirants that the door to membership in the Euro-Atlantic community of nations remains open and that countries in the Western Balkans are free to choose their own future and select their own partners without outside interference or intimidation,” the statement said.

The treaty signing comes after the Senate on Mach 28 votedoverwhelmingly to back Montenegro joining NATO.

Republican Sens. Rand Paul (Ky.) and Mike Lee (Utah) were the only two Senate lawmakers to oppose the treaty, arguing that allowing Montenegro to join does little to help U.S. national security interests.

“I don’t see how the accession of Montenegro, a country with the population smaller than most congressional districts and a military smaller than the police force of the District of Columbia, is beneficial enough that we should share an agreement for collective defense,” Lee said at the time.

Paul also blocked a vote on the treaty earlier in March, setting off Senate Armed Services Committee Chairman John McCain (R-Ariz.).

McCain blasted Paul on the Senate floor at the time, saying that the “senator from Kentucky is now working for [Russian President] Vladimir Putin.”

But lawmakers from both parties stressed that the move will help push back against a resurgent Russia.

All current NATO members will also have to approve Montenegro’s membership.