Montenegro's NATO membership will send a strong signal of commitment to regional security, secretary general Jens Stoltenberg said Thursday, amid growing Western concern about Russian influence in the Balkans.

"Your NATO membership will send a clear signal of stability and security in the whole region, which is the basis for prosperity," Stoltenberg told Montenegro Prime Minister Dusko Markovic at the alliance's headquarters in Brussels.

"NATO membership will strengthen Montenegro's sovereignty," he added, apparently a reference to recent turmoil involving pro-Russian groups in Montenegro who oppose joining the alliance.

NATO signed the accession agreement with Montenegro last year, paving the way for the country to become its 29th member at a time of increasing tension between the West and a more assertive Russia.

Moscow condemned the move as another instance of NATO's encroachment in its backyard, helping fan the opposition groups that demanded a referendum on the issue.

Stoltenberg said NATO and Montenegro had worked together closely for many years, and he was "looking forward to take that cooperation to a new level when Montenegro becomes a full member."

Twenty-one of the 28 NATO allies have ratified Montenegro's accession, and France was expected to do so on Thursday, he said.

Asked about US ratification and concerns that President Donald Trump's more sceptical view of NATO could cause delay, Stoltenberg said he was not worried.

"I am confident that the accession protocol will be ratified by the Senate. It has already passed the Senate Foreign Relations Committee and it has strong bipartisan support," he said.

Markovic also said he was confident there would be no problems.

"I would say this tension over US ratification is encouraged by those who are against it, but we are absolutely certain it will take place very soon," he said.

Among other states of the former Yugoslavia, Croatia and Slovenia have joined NATO, along with many East European countries once ruled from Moscow.