Pence told leaders of the Western Balkans that Russia was working to destabilize the region and that they needed to be "resolute and uncompromising in the face of aggression from an unpredictable country that casts a shadow from the east".

"As you well know, Russia continues to seek to redraw international borders by force. And here, in the Western Balkans, Russia has worked to destabilize the region, undermine your democracies, and divide you from each other and from the rest of Europe," he said.

Read more: US VP Mike Pence in Estonia raises prospect of deploying Patriot missiles

Russia's worrying behavior

Pence was speaking at the Adriatic Charter Summit in the North Atlantic Treaty Organization's (NATO's) newest member. It was the last stop in a tour through eastern Europe that aimed to reassure allies worried by Russia's behavior.

Pence accused "Moscow-backed agents" of trying to attack Montenegro's parliament and assassinate its prime minister at the time, Milo Djukanovic, during an election in October last year. He said the suspected power grab aimed "to dissuade the Montenegrin people from entering our NATO alliance."

Read more: Kremlin denies Montenegro assassination plot

Read more: 'Europe is the powder keg - the Balkans are the fuse'

Assasination plot

Pence's comments stepped up US accusations over the incident, after the White House said in April that it had seen "credible reports of Russian support for an attempted election-day attack" in the ex-Yugoslav republic.

A group of Serbians were arrested over the alleged coup plot and 14 suspects were scheduled to face trial in the Montenegrin capital, including two Russians in absentia.

Leaders from Albania, Bosnia, Croatia, Macedonia, Kosovo, Serbia and Slovenia attended Wednesday's summit to discuss advancing their Euro-Atlantic aspirations and reforms.

Read more: US VP Mike Pence embarks on European reassurance mission

Read more: The Balkans: From Yugoslav wars to an ever-tense peace

NATO's doors are open

Pence told the leaders that NATO's door would always be open "for those European countries that share our values, contribute to the common defense, and strive to achieve security, prosperity, and freedom for their people."

Montenegro joined NATO in early June, angering Russia, which considers it to be part of its historic sphere of influence and a traditional Slavic ally.

Read more: Montenegro 'entered the West' through NATO

Serbia is now Russia's sole ally in the Balkans, although Belgrade officially says it wants to join the European Union. Serbia has been strengthening military ties with Moscow, while maintaining a partnership relationship with NATO.

Earlier in his tour, Pence pledged support for the former Soviet republic of Georgia and met with the presidents of three NATO countries - Estonia, Latvia and Lithuania - and proclaimed "an attack on one of us is an attack on us all."

aw/jr (AP, AFP, Reuters)