WASHINGTON -- U.S. President Donald Trump said Thursday that Russian violations make it untenable for the U.S. to stay in a treaty that permits 30-plus nations to conduct observation flights over each other's territory, but he hinted it's possible the U.S. will reconsider the decision.

The Open Skies Treaty that governs the unarmed overflights was initially set up to promote trust and avert conflict between the U.S. and Russia. The Trump administration informed other members of the treaty that it will pull out in six months because Russia is violating the pact and imagery collected during the flights can be obtained quickly at less cost from U.S. or commercial satellites.

"Russia didn't adhere to the treaty. So until they adhere, we will pull out, but there's a very good chance we'll make a new agreement or do something to put that agreement back together," Trump told reporters at the White House before leaving for Michigan.

"So I think what's going to happen is we're going to pull out and they (the Russians) are going to come back and want to make a deal," Trump said. He added: "I think something very positive will work."

The U.S. announcement that it plans to leave the treaty is expected to strain relations with Moscow and upset some members of Congress and European allies, which benefit from the imagery collected by Open Skies flights conducted by the U.S.

In Moscow, Russian Deputy Foreign Minister Alexander Grushko criticized the U.S. decision.

"Our position is absolutely clear and is invariable: The withdrawal of the US from this treaty will come as yet another blow to the system of military security in Europe, which is already weakened by the previous moves by the administration," Grushko told state news agency Tass.

Trump's national security adviser Robert O'Brien said the president has made clear that the United States will not remain a party to international agreements being violated by the other parties and are no longer in America's interests. He said Russian violations prompted Trump last year to pull out of the 1987 nuclear arms treaty with Russia.

That treaty, signed by President Ronald Reagan and Soviet leader Mikhail Gorbachev, banned production, testing and deployment of land-based cruise and ballistic missiles with a range of 500 to 5,500 kilometres.

Trump's decision to exit the Open Skies Treaty also raises questions about his commitment to extending or renegotiating the New START treaty, which expires early next year. New START, the only remaining treaty constraining the U.S. and Russian nuclear arsenals, imposes limits on the number of U.S. and Russian long-range nuclear warheads and launchers. Russia has offered to extend the treaty, but Trump is holding out in hopes of negotiating a three-way agreement with the U.S. and China.

"We look forward to negotiating with both Russia and China on a new arms control framework that moves beyond the Cold War constructs of the past and helps keep the entire world safe.," O'Brien said in a statement.

President Dwight Eisenhower first proposed the United States and the former Soviet Union allow aerial reconnaissance flights over each other's territory in July 1955. At first, Moscow rejected the idea, but President George H.W. Bush revived it in May 1989, and the treaty entered into force in January 2002. Thirty-four nations have signed it; Kyrgyzstan has signed but not ratified it.

More than 1,500 flights have been conducted under the treaty, aimed at fostering transparency about military activity and helping monitor arms control and other agreements. Each nation in the treaty agrees to make all its territory available for surveillance flights and share all the imagery collected, yet Russia has restricted flights over certain areas.

Alexandra Bell, a former State Department official and currently the senior policy director at the nonpartisan non-profit Center for Arms Control and Non-Proliferation, said withdrawal from Open Skies will rub allies the wrong way.

"I absolutely cannot see a single upside to abandoning this treaty against the advice and wishes of our allies, other than for the people who never liked this treaty and don't like the idea of the transparency and openness the treaty provides," Bell said.

The U.S. has been working on a proposal to share with partners and allies imagery the U.S. would have shared from its Open Skies flights, said senior administration officials, who spoke on condition of anonymity to explain Trump's decision.

Last month, top Democrats on the Foreign Affairs and Armed Services committees in the House and the Senate wrote to Trump accusing the president of "ramming" a withdrawal from the treaty as the world grapples with COVID-19. They said it would undermine U.S. alliances with European allies who rely on the treaty to keep Russia accountable for its military activities in the region.

This month, 16 former senior European military and defence officials signed a statement supporting the treaty, saying a U.S. withdrawal would be a blow to global security and further undermine the international arms control agreements.

If the U.S. and Russia exit, all U.S. and Russian territory would be off limits to the overflights. That prompts arms control experts like Steve Pifer at the Brookings Institution to ask "What would be the point?"' On the other hand, he said, Moscow could opt to stay in the treaty, which would at least allow it to continue overflights of American facilities in Europe.

Senior administration officials said Russian violations include restricting flights over Moscow and Chechnya and near Abkhazia and South Ossetia. Russian restrictions also make it difficult to conduct observation in the Kaliningrad, a Russian enclave sandwiched between Lithuania and Poland that is home to Russia's Baltic fleet, they said.

Russia uses illegal overflight restrictions along the Georgian border in support of its propaganda narrative that the Russian-occupied enclaves of Georgia are independent countries.

Associated Press Diplomatic Writer Matthew Lee and AP National Security Writer Robert Burns in Washington and James Heintz in Moscow contributed to this report.