NATO will hold a meeting with the Russian ambassador to the US-led military alliance on Wednesday, just one day after a second emergency gathering on the Ukraine crisis.

"There will be an NRC (NATO-Russia Council meeting) tomorrow at ambassadorial level. We expect it will take place in the afternoon," a NATO spokesman said Tuesday.

NATO head Anders Fogh Rasmussen had suggested the meeting Monday.

The NRC is the official forum for discussions and contacts between NATO and its one-time Cold War foe. It is regularly convened, for example during alliance foreign minister gatherings.

After the fall of communism in the 1990s, Russia established ties with NATO and has a wide range of contacts with it, over Afghanistan for example, and combating terrorism.

Those ties have been badly strained, however, by the Ukraine crisis and Russia's intervention in Crimea, home to a large Russian-speaking population and its Black Sea fleet.

On Tuesday, the 28 NATO member ambassadors met to discuss the situation after Poland requested "Article 4" consultations with its allies, citing a threat to its security.

This meeting was convened because "developments in and around Ukraine are seen to constitute a threat to neighbouring allied countries and having direct and serious implications for the security and stability of the Euro-Atlantic area," Rasmussen said on Monday.

"Under Article 4 of the treaty, any ally can request consultations whenever, in the opinion of any of them, their territorial integrity, political independence or security is threatened," a statement said.

Rasmussen said Tuesday's meeting agreed that "Russia continues to violate Ukraine's sovereignty and territorial integrity, and does not fulfil its international commitments."

"These developments have direct and serious implications for the security and stability of the Euro-Atlantic area," he added.

As a result, the NATO allies agreed "to intensify our rigorous and ongoing assessment of the implications of this crisis for Alliance security".

Diplomats said there was no discussion of any military operational planning, with the meeting focused on the need to review the situation regularly and to keep member states fully informed.

Article 4 meetings are quite rare but were held most recently after Turkey feared a spillover from the bloody conflict in Syria.