Foreign ministers from the 29 allies will meet Ukraine's foreign minister on Tuesday to discuss last week's incident in the Sea of Azov, in which Russian forces seized three Ukrainian ships and 24 sailors.

Ministers will also tackle the Intermediate Range Nuclear Forces (INF) treaty, a major Cold War accord which Washington has announced it will abandon in response to Russian violations, with Stoltenberg promising a "measured, proportionate" response.

European leaders used weekend G20 meetings in Buenos Aires to press Russian President Vladimir Putin over the Ukraine spat, but Western powers are wary of taking steps that might inflame the situation.

Ukraine's President Petro Poroshenko looks likely to be disappointed in his call for NATO to deploy naval vessels to the Sea of Azov, a gulf of the Black Sea between Crimea and the Ukrainian and Russian mainlands.

Stoltenberg repeated a call for Russia to release the sailors and ships and allow unhindered access to Ukrainian ports in the Sea of Azov.

He said he expected ministers to "convey a very clear message to Russia" but refused to offer any fresh practical support measures for Kiev, simply outlining the ways NATO has raised its profile in the Black Sea.

"NATO has already increased our presence in the Black Sea -- significantly more days with NATO ships at sea this year than last year, and we have more air policing... more presences in the Black Sea in general," Stoltenberg told reporters.

"We will of course closely monitor the situation in that region also in light of what we saw a few days ago."

A European diplomat said that no boats would be sent in response to the Ukrainian request and that the idea had not even been formally suggested at NATO. Ukraine is a "partner country" for NATO but not a full member.

Last week's incident was the first open military confrontation between Russia and Ukraine since 2014, when Moscow annexed the Crimea peninsula and pro-Kremlin separatists in the east entered into conflict with Ukrainian forces.

- More time on INF? -

Western powers say Russia has developed and deployed a missile system that breaches the 1987 accord, which banned ground-launched missiles with a range of 500 and 5,500 kilometres.

Stoltenberg said the mobile, nuclear-capable Russian missiles could hit European cities "with little or no warning". He said NATO would not "mirror what Russia does plane by plane, missile by missile" but would respond in its own way to assure the defence of the alliance.

There have been suggestions that US Secretary of State Mike Pompeo may use the NATO meeting to formally end Washington's involvement in the INF, but on Monday his boss, President Donald Trump, proposed talks with Russia and China to halt what he called an "uncontrollable" arms race.

And diplomats said they may give Russia more time to come back into compliance with the INF treaty -- though the US has been fruitlessly pressing Moscow to do so for several years.

"The Russians must understand that they must respect their commitments and that they have very little time left to get back in line," one diplomat said.

Another diplomat said the ministerial meeting would likely agree an "exit schedule" for the US to quit the INF treaty, indicating that it "won't be abrupt".