PODGORICA, Montenegro — U.S. Senator John McCain congratulated Montenegro on Wednesday for its upcoming NATO membership and blasted Russia for its alleged attempts to interfere in the Balkans and more widely in Europe.

McCain, chairman of the Senate Armed Services Committee, is on a tour of the western Balkans, the war-weary region where Russia has been vying for increased military, political and economic influence.

"The behavior on the part of Russians, throughout the region and the world, is not acceptable," McCain, one of the strongest supporters of NATO expansion in Europe, said in Montenegro.

The Senate backed Montenegro's accession to NATO last month, a message that the U.S. will push back against Russian efforts to increase its influence in Europe.  President Donald Trump signed the ratification on Tuesday.

NATO invited tiny Montenegro — a traditional Russian ally — to join the military alliance as its 29th member in December 2015.

"I believe the Russian behavior requires our solidarity and our strength in order to preserve principles and fundamentals of democracy," McCain, who was a prisoner of war during the Vietnam War as an American airman, said.

Montenegrin authorities say they thwarted an October attempt by Serb and Russian nationalists to assassinate then-prime minister Milo Djukanovic to stop Montenegro from joining NATO.

In an interview with The Associated Press, Djukanovic has accused the Kremlin's secret services of standing behind the alleged coup attempt on election day.

McCain drew a parallel between the events in Montenegro and the alleged Russian interference in the U.S. presidential election.

"There's no evidence that they succeeded," McCain said, referring to the election that put Trump into office. "But there is also evidence that Russians are trying to undermine the democratic process in other parts of the world."

At a joint press conference with McCain, Montenegro Defense Minister Predrag Boskovic said a Euro-Atlantic partnership "is the only way for this region to preserve security and stability."

"This region will need special attention in the years after we enter NATO," he said.

Dusan Stojanovic contributed from Belgrade, Serbia.