US President

"Today, President Donald J. Trump signed the United States instrument of ratification of the Protocol for Montenegro's accession to the North Atlantic Treaty Organization (NATO)," the White House said in a statement.

The decision had largely been expected, despite Trump's sometimes mercurial decision-making and harsh words for the military alliance.

It will further strain US relations with Russia, which has described Montenegro's accession as a "provocation."

Under Russian leader Vladimir Putin, the Kremlin has long seen the Balkans as inside its "sphere of influence."

Trump's approval comes on the eve of his meeting with NATO Secretary General Jens Stoltenberg at the White House.

The Republican president has repeatedly called the United States' commitment to the alliance into question, demanding allies pay more toward collective defense.

But he has tempered his most incendiary rhetoric since coming to office and is likely to attend a NATO summit in Brussels on May 25.