The United States said on Sunday that it had officially withdrawn from the Open Skies Treaty, in yet another move to abandon a major international arms control agreement.

"Today marks six months since the United States submitted our notice of withdrawal from the Treaty on Open Skies," National Security Advisor Robert O'Brien said in a tweet.

The Trump administration submitted the notice of its decision to withdraw from the treaty in May, and the Sunday announcement marked the end of a six-month notification process.

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The treaty, which became effective in 2002, allows its 34 State Parties to conduct short-notice, unarmed reconnaissance flights over the others' entire territories to collect data on military forces and activities.

The United States and Russia have blamed each other for noncompliance with the treaty. Washington and Moscow have each put a few limits on flights over their territories – Hawaii and some other U.S. bases have been off-limits, as has Kaliningrad, among others, according to media reports.

The treaty is aimed at building confidence and familiarity among State Parties through their participation in the overflights. By 2019, over 1,500 Open Skies flights had been conducted since the deal entered into force, according to media reports.

Some experts noted that a unilateral U.S. exit from the treaty would undermine U.S. commitment to its NATO allies.

Moscow will seek firm guarantees that the states remaining in the Treaty on Open Skies fulfill their obligations, the Russian Foreign Ministry said Sunday in connection with the U.S. withdrawal from the treaty.

"After withdrawing from the Treaty on Open Skies, the U.S. side expects that its allies, on the one hand, will hinder Russian observation flights above U.S. military facilities in Europe, and, on the other hand, share with Washington their photographic materials of Russian territory," it said in a statement.

"Of course, this is unacceptable for Russia," it added.

"We will seek firm guarantees that the states remaining in the Treaty on Open Skies will fulfill their obligations, firstly, on ensuring the possibility of observing their entire territory and, secondly, on ensuring that the materials of observation flights will not be transferred to third countries that are not participants of the treaty," it said.

China on Monday expressed deep regret over the U.S. decision to withdraw from the treaty regardless of opposition from the international community.  

Foreign Ministry spokesperson Zhao Lijian said at a regular press briefing that the decision will damage military mutual trust and transparency between countries in the region and negatively affect the international arms control and disarmament process. 

Wantonly withdrawing from international treaties is not a proper practice for a major country, Zhao said.

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In May, U.S. allies in Europe voiced their regret over Washington's plan to pull out of the treaty. 

"We regret the announcement by the government of the United States of its intention to withdraw from the Open Skies Treaty, even though we share its preoccupations about the implementation of the treaty's provisions by the Russian Federation," a joint statement from the foreign ministries of France, Germany, Belgium, Spain, Finland, Italy, Luxembourg, the Netherlands, the Czech Republic and Sweden said. 

Calling the treaty "crucial," the 10 European signatories said they would continue implementing the pact and that it "remains functional and useful." 

Apart from the Open Skies Treaty, the Trump administration has also pulled the U.S. out of the Intermediate-range Nuclear Forces Treaty with Russia and refused to renew the New START treaty, the last agreement limiting U.S. and Russian strategic nuclear weapons, which is due to expire next February. 

The U.S. has also withdrawn from the 2015 Paris agreement on climate change, the 2015 Iran nuclear deal, the Trans-Pacific Partnership, UNESCO, the UN Human Rights Council and the World Health Organization. 

(With input from Xinhua) 

(Cover: U.S. Air Force Thunderbirds perform aerobatics during the 57th annual Abbotsford International Airshow in Abbotsford, Canada, August 11, 2019. /Xinhua)