Western allies are hoping to convince Washington to reverse the decision, which Trump said was due to Moscow not honouring the defence agreement.

NATO Secretary General Jens Stoltenberg said the US decision to quit the agreement will not come into effect for six months, leaving Moscow time to change course.

"All NATO allies are in full compliance with all provisions of the treaty," Stoltenberg said.

"Russia has for many years imposed flight restrictions inconsistent with the treaty, including flight limitations, over Kaliningrad and restricting flights in Russia near its border with Georgia.

EU foreign policy chief Josep Borrell said he regretted the US decision, calling the Open Skies treaty "a key element of our arms-control architecture" which serves as "a vital confidence and security-building measure."

He called on Washington to reconsider and for Moscow to "return immediately to the full implementation of the Treaty".

Earlier a group of 10 European nations said in a joint statement they regretted Trump's threat -- his latest in a string of withdrawals from international agreements.

- 'Entrenched Cold War mentality' -

The pact allows its nearly three dozen signatories to carry out short-notice flights over one another's territory to monitor potential military operations.

Members include countries across Europe and the former Soviet Union, as well as the United States and Canada.

Trump said Thursday he would would pull the US out, alleging Moscow had not adhered to its commitments under the pact.

The US accuses Russia of blocking flights over certain sites and forbidding surveys of military exercises, normally allowed under Open Skies.

Moscow said on Friday it would continue observing the treaty even if the US pulls out.

"As long as the treaty is in force, we intend to fully follow all the rights and obligations that apply to us from this treaty," Deputy Foreign Minister Alexander Grushko told the RIA Novosti news agency.

Fellow Deputy Foreign Minister Sergei Ryabkov denounced the "absolutely unacceptable" conditions set by the Washington, accusing the US of sowing "discord and uncertainty"

The foreign ministry accused the US of pointing the finger at Russia "to camouflage their destructive actions".

Grushko warned that the US pullout would damage European security and harm the interests of US allies.

China, which is not a party to the treaty, expressed "deep regret" over the US move, calling it a "display of the United States' entrenched Cold War mentality'.

The Europeans said they would work to resolve "outstanding questions" with Moscow, including "unjustified restrictions" imposed on flights over Kaliningrad -- a Russian exclave bordered by Poland and Lithuania.

"We continue to urge Russia to lift these restrictions," they said.

China, which is not a party to the treaty, expressed "deep regret" over the US move, calling it a "display of the United States' entrenched Cold War mentality".

- 'Security and peace' -

The withdrawal "will have a negative impact on the international arms control and disarmament process," foreign ministry spokesman Zhao Lijian said.

US Secretary of State Mike Pompeo said the United States could reconsider "should Russia return to full compliance with the Treaty."

Open Skies is the third important military pact that Trump has withdrawn from since coming to office in January 2017.

He halso dropped the 2015 JCPOA agreement to prevent Iran from advancing its nuclear weapons program and the 1988 Intermediate Range Nuclear Forces Treaty with Russia.

In both cases, Trump accused the other side of violating treaty requirements.

The latest move adds to question marks over New START, a pact that limits the number of nuclear missiles the US and Russia can deploy, which is due for renewal by early 2021.

The Open Skies treaty carried more political than military weight, according to Corentin Brustlein of the Paris-based French Institute of International Relations.

Large spy agencies do not need "open skies" to gather information on other countries' military activities, he told AFP.

"But the information gathered under Open Skies is shareable and shared," he said, including with signatory states that do not have strong intelligence agencies of their own.

"The only negative consequences of this withdrawal will be felt by allies of the United States... It is yet another demonstration of what little regard the US administration has for Europe's security concerns."

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