PODGORICA - Montenegro's accession to NATO is vital for regional stability and the joint effort of the Western allies to resist a resurgent Russia, U.S. Senator John McCain said on Wednesday.

The visit of McCain, the Republican chairman of the influential Senate Armed Services Committee, to Montenegro comes a day after U.S. President Donald Trump signed the U.S. ratification for the tiny Adriatic republic's accession to NATO, which Russia opposed.

"There is also evidence that Russians are trying to undermine the democratic process in other parts of the world," McCain told reporters.

"I believe the Russian behavior requires our solidarity and our strength, in order to preserve principles and fundamentals of democracy," McCain said.

Montenegro has population of 650,000 and a military of only 2,000, but it is strategically positioned along the Adriatic coast and surrounded by NATO members or hopefuls, except Serbia which maintains a military neutrality.

The U.S. Senate backed Montenegro's bid to join the alliance Last month.

"The behavior on the part of the Russians throughout the region and the world is not acceptable in these times," McCain told a news conference after meeting Montenegrin Defense Minister Petar Boskovic.

McCain's remarks also coincided with the visit of the U.S. Secretary of State Rex Tillerson to Russia as relations between the two superpowers soured over the course of the war in Syria.

Backers of Montenegrin accession to NATO say it is important to support new members to promote Western values and push back against Moscow.

Montenegrin officials said Russia was behind an alleged plot last October aimed at halting the integration of the former Yugoslav republic into NATO and bringing the opposition to power. Moscow dismissed the allegations.

All 28 NATO members must ratify Montenegro's accession in order for the country to join the alliance. Washington was among the last to do so.