A sovereign, independent and stable Ukraine is essential to security in Europe and beyond, NATO said Wednesday as tensions between pro- and anti-Russian factions stoked fears the country could break apart.

Such a Ukraine, "firmly committed to democracy and the rule of law, is key to Euro-Atlantic security," NATO said in a statement agreed by member state defence ministers.

In line with current agreements with Kiev, "NATO allies will continue to support Ukrainian sovereignty and independence, territorial integrity, democratic development, and the principle of inviolability of frontiers, as key factors of stability and security in Central and Eastern Europe and on the continent as a whole."

"Ukraine is the most important security issue in Europe today," NATO head Anders Fogh Rasmussen told a press conference separately.

NATO's statement was a "message of solidarity and support" for the Ukraine people and it was "of the utmost importance" that it was made, Rasmussen said.

It came amid growing concern about Ukraine's future after the ouster of pro-Russian president Viktor Yanukovych.

Brawls broke out Wednesday in Crimea between demonstrators wanting to stick with Kiev and opponents backing closer links with former Cold War master Russia.

Crimea is home to Russia's Black Sea fleet and tensions mounted further after President Vladimir Putin ordered military preparedness checks in western Russia.

The drill -- similar to one last year in the east -- involves army, navy and airforce troops based in the western military district, a vast territory bordering Ukraine, Belarus, the Baltic states, Finland and the Arctic.

Rasmussen said Russia had informed the military alliance of the exercise but that he himself had only been told in the afternoon as he had been in meetings.

"I suppose Russia has lived up to all its obligations" on notification, he said.

Interim leaders in Kiev meanwhile named a new government which is expected to take the country closer to the west after months of bloody unrest sparked when Yanukovych ditched an EU association accord under pressure from Moscow.

- 'Close and long-standing partner' -

Arriving for the first day of a two-day NATO defence ministers meeting, Rasmussen earlier promised help for reform in Ukraine, "a close and long-standing partner to NATO."

"We stand ready to continue assisting Ukraine in its democratic reforms," Rasmussen said.

Asked if he had been in contact with Russia over developments in Ukraine, he did not answer directly.

"Let me stress that it's for the Ukrainian people to determine what should be the future of their country," he said.

"We take it for granted that all nations respect the sovereignty and independence and territorial integrity of Ukraine."

"This is a message that we have also conveyed to whom it may concern," he said, without naming Russia.

Given deep political and economic ties, plus the importance of Crimea for its navy, there has been some speculation Moscow could intervene directly to secure the base in the event the country breaks apart.

German Defence Minister Ursula von der Leyen stressed the importance of maintaining contacts with Russia in the crisis.

"One thing is very clear: it is not only in the interest of Germany, but of NATO and Russia too, that Ukraine finds its way back to stability," von der Leyen said.

"The country must not fall apart. Russia needs to be involved, there won't be a solution without Russia," she said.

The 28 NATO member state defence ministers were due to meet a Ukrainian delegation on Thursday.

In 1997, NATO set up a joint commission with Ukraine to oversee relations and in 2008 agreed that Kiev could ultimately be considered for membership of the Cold War era alliance.