The idea that has upended President Barack Obama's push for a military attack on Syria—tossed out as an aside by his secretary of state at a news conference—had been discussed with Russia over the past year and more frequently in recent weeks, according to a U.S. account of the discussions.

Secretary of State John Kerry acted on his own, without White House direction, when he suggested on Monday that Syria could avert a U.S. attack if it gave up its chemical weapons.

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