FILE PHOTO - A NATO flag flies at the Alliance's headquarters in Brussels, March 2, 2014. REUTERS/Yves Herman/File Photo

WASHINGTON (Reuters) - U.S. President Donald Trump on Tuesday signed the U.S. instrument of ratification for Montenegro’s accession to NATO, the White House said in a statement.

Last month the Senate overwhelmingly backed the expansion of NATO to allow Montenegro to join the alliance, hoping to send a message that the United States will push back against Russian efforts to increase its influence in Europe.