Open Skies was agreed just after the Cold War to allow signatories to avoid nasty surprises or unfounded suspicions by monitoring rival militaries.

But Trump said in May that Moscow had not stuck to its commitments under the pact, which was designed to improve confidence between the superpowers.

"The US withdrawal took effect on November 22, 2020, and the United States is no longer a State Party to the Treaty on Open Skies," the US State Department said in a statement Sunday.

The treaty was signed in 1992 and came into force in 2002, allowing 35 signatory countries, including the United States and Russia, to fly unarmed surveillance flights over each other's territory.

Moscow and Washington had often accused each other of breaching its terms, and in May the Trump administration said it gave the required six-month notice to leave the treaty.

The United States under Trump has also pulled out of the 2015 Iran nuclear deal as well as the Paris accord against climate change.

President-elect Joe Biden, who assumes the Oval Office in January, has vowed that the US would rejoin on his first day in the White House.