Fifteen years have passed since the Czech Republic, Hungary and Poland joined the NATO military alliance, a decision the ex-Soviet satellites now see as fortuitous given today's Ukraine standoff between Russia and the West.

"NATO is the safest alliance Poland ever joined," Polish Prime Minister Donald Tusk said Monday, hours before NATO announced it was deploying reconnaissance aircraft in his country and Romania as part of efforts to monitor the crisis in neighbouring Ukraine.

NATO's 1990s overture to the East marked the beginning of the alliance's enlargement to former members of the now-defunct Soviet-led Warsaw Pact.

After a decade of tough negotiations, the Czech Republic and Ukraine's neighbours Hungary and Poland joined the alliance on March 12, 1999.

This paved the way for a host of other ex-communist Central European states -- Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia -- to follow suit in 2004, thus erasing the divisions of the Cold War.

"If Poland weren't in NATO, then the Baltic states (Estonia, Latvia and Lithuania) certainly wouldn't be in it either," former Polish president Aleksander Kwasniewski said Sunday.

"There was a kind of security vacuum in this part of the world, which could have been filled by others," he said, adding that without NATO membership the Ukrainian conflict would pose "an extremely serious threat" for Poland and the Baltics.

"But today we're under the NATO umbrella and can sleep more soundly."

Sixty-two percent of Poles say they are happy with NATO membership, while only four percent disapprove, according to a February poll from the CBOS institute.

- A new Cold War? -

The Ukraine crisis has raised the spectre of the Cold War, with the leaders of Poland, the Czech Republic, Hungary and Slovakia comparing Russia's actions in Ukraine to Soviet crackdowns at home during that era.

NATO also held rare emergency talks after Poland requested consultations under Article 4 of the alliance treaty, which any member can do when they believe their territorial integrity, political independence or security is threatened.

To reassure its eastern European NATO allies, the United States stepped up a joint training exercise with Poland and strengthened the alliance's air patrols over the Baltic states.

A dozen F-16 fighter jets and 300 US service personnel will descend on Poland by Thursday to take part in the exercise, originally planned to be smaller but increased and pushed forward because of the "tense political situation" in Ukraine, Polish defence ministry spokesman Jacek Sonta told AFP.

While Russian President Vladimir Putin says the breakaway Crimea peninsula has the right to join his country, Ukraine has sought US help to end Moscow's "aggression" in the strategic region, where it has effectively seized control.

Washington, along with London and Moscow, pledged to respect Ukraine's territorial integrity in a 1994 agreement under which the ex-Soviet republic agreed to give up its nuclear weapons.

"NATO today is no longer the same organisation that fought the USSR," said security expert and former Czech military intelligence chief Andor Sandor.

"It's no longer the alliance that we joined 15 years ago, but I continue to think it's in our best interest security-wise.

"Membership allowed our army to learn many things. We've met the Western standard in a lot of ways, and today many soldiers have completed missions, trained, held positions," he told AFP.

Hungary is likewise satisfied with its decision to join, according to defence expert Istvan Balogh from the Hungarian Institute of International Affairs, the foreign ministry's think-tank.

"NATO membership has definitely been a net benefit to Hungary. It has anchored us in the transatlantic community," he told AFP.

"The combat experience we got out of Afghanistan, seeing how a 21st-century military environment works, has also contributed to developing the Hungarian military."