A US deadline for Russia to come back into compliance with the Intermediate-Range Nuclear Forces (INF) treaty expires on Saturday but Secretary of State Mike Pompeo will reportedly announce the pullout as early as Friday.

Western capitals say a new Russian medium-range missile system breaches the terms of the 1987 treaty and puts European cities at risk.

Heiko Maas, the German foreign minister who has shuttled between Moscow and Washington in recent weeks trying to save the INF treaty, said Russia was still in breach.

"We must note that the INF treaty has been violated by the Russian side and that the appeals of the last 60 days for more transparency and more information have come to nothing," Maas said as he arrived for talks with fellow EU foreign ministers in Bucharest.

"A treaty to which two contracting states belong and which has been violated by one side has de facto been invalidated".

Lithuania and Latvia -- Baltic states keenly aware of the threat from their giant neighbour Russia -- backed Washington's expected withdrawal.

"Treaties are important if parties are complying to the treaties. If there are breaches, the value of the treaty is questioned," Lithuanian Foreign Minister Linas Linkevicius said, adding there was "clear evidence" of Russia breaching the treaty.

- History's taught 'a clear lesson' -

The INF accord, signed by then US president Ronald Reagan and Soviet leader Mikhail Gorbachev, banned ground-launched missiles with a range of 500 to 5,500 kilometres and ended a dangerous build-up of warheads in Europe.

Russia on Friday said it regretted Washington's planned exit from the treaty and expected to receive formal notice from the United States shortly.

"We all regret that in the coming days this decision will most likely be implemented," Kremlin spokesman Dmitry Peskov told journalists.

"A decision to move to break the treaty was made in Washington a long time ago."

Russian President Vladimir Putin has warned of a new arms race if the treaty collapses -- a prospect that has alarmed some European nations.

EU diplomatic chief Federica Mogherini called for the treaty to be preserved "with full compliance" from both sides.

"What we definitely don't want to see is our continent going back to being a battlefield or place where other superpowers confront themselves -- this belongs to a faraway history," she said.

Hungary's Foreign Minister Peter Szijjarto said history had taught his country "a very clear lesson".

"This clear lesson is whenever there was a conflict between east and west, we central Europeans always lost," he said.

"We as Hungary I don't think we have too much leverage in this issue. We can just cross fingers for a more pragmatic cooperation between east and west."

- Another treaty threatened -

To add further to Europe's worries, Moscow's top INF negotiator, deputy foreign minister Sergei Ryabkov, warned that after the collapse of the treaty another key arms control agreement -- the New START treaty -- could follow.

That agreement, which caps the number of nuclear warheads held by Washington and Moscow, expires in 2021 and Ryabkov said there was a "big question" over what would happen next.

Pompeo announced in December that the US would start the six-month process to quit the INF if Russia did not withdraw its 9M729 missile system by February 2, using the period to start work on new missiles.

While all 29 NATO allies have endorsed the US view that Russia is in breach of the treaty, differences have emerged over how to respond.

Belgian Foreign Minister Didier Reynders said withdrawing was "not the right response", arguing that leaving the treaty would not "succeed in putting more pressure" on Moscow.

Asked if Russia was to blame for the INF being in jeopardy, Karin Kneissl, the Austrian foreign minister, who is close to Putin, said it was "a bit more complicated" than that, pointing to Moscow's concerns about China, which is not bound by the treaty.

NATO chief Jens Stoltenberg has said military commanders will begin preparing for "a world without an INF treaty" but insisted the alliance was still committed to arms reduction.

Russia insists the missile system complies with the treaty, and displayed it to foreign military officials and media last week in a bid to allay concerns.