WASHINGTON - President Donald Trump said Thursday he is pulling the United States out of the 18-year-old international Open Skies treaty allowing surveillance flights over other countries because Russia has been violating it.

The U.S. began notifying the other 33 signatories to the accord that it was giving the required six months' notice to leave.

At the White House, Trump accused Moscow of ignoring terms of the treaty. Consequently, Trump told reporters, "We're not going to adhere to it either," although he held out hope that some new agreement might be reached.

The treaty has allowed the 34 countries to conduct surveillance flights over each other's territory to look at military installations, an effort aimed at international peacekeeping.

But the U.S. contends that Moscow has been violating the treaty by blocking it from conducting flights over the Baltic Sea city of Kaliningrad and Russia's southern border near Georgia, both of which are permitted by the agreement.

U.S. Defense Secretary Mark Esper told the Senate Armed Services Committee in March that Russia has been "cheating for many years."

Russia has denied violating the treaty.

The Open Skies withdrawal is Trump's latest move during his three-and-a-half-year presidency to remove the U.S. from international agreements enacted by previous presidents that he considers unfair to American interests.

Last year, Washington withdrew from the Intermediate-Range Nuclear Forces Treaty with Russia, and earlier had abandoned the international Paris climate control accord, a trade treaty with Pacific Rim nations and a deal with Iran to restrain its development of nuclear weapons.

The concept of the Open Skies treaty was first advanced in the decade after the end of World War Two by President Dwight Eisenhower. The Soviet Union balked at the idea at the time, but the U.S. again pushed for a pact in 1989 under President George H.W. Bush. It was finally adopted by the required 20 countries in January 2002.