“What we were shown before and recently by our American partners, as well as by the British and French, does not convince us at all,” Mr. Lavrov said on Monday. “There are no facts, there is simply talk about ‘what we definitely know.’ But when you ask for more detailed evidence, they say that it is all classified, therefore it cannot be shown to us. This means there are not such facts to encourage international cooperation.”

Mr. Lavrov also took a direct jab at Mr. Kerry. “It is very strange to hear, when we recently discussed the issue, my good colleague, U.S. Secretary of State John Kerry, say that the American side had produced irrefutable evidence for Russia of the Assad regime using chemical weapons, and then claiming that Russians deliberately refused to recognize the fact.”

Later, at a news conference in Moscow with the South African foreign minister, Mr. Lavrov said Russia would insist that the United States comply with international agreements and not attack Syria without the consent of the Security Council.

“If someone tries to make gross violations of international law a norm, then we will create chaos,” Mr. Lavrov warned. “We will create a situation where the U.N. Charter and the principles under which all the nations of the world have signed up, including the principle of unanimous agreement of the permanent members of the U.N. Security Council, the so-called right of veto, which the United States insisted on — then all of these principles will simply collapse.”

In a meeting with Mr. Putin ahead of the autumn legislative session, the leaders of the Russian Parliament said that they planned to reach out to their counterparts in the United States Congress to explain their opposition to a military strike against the Assad government and to urge them to reject Mr. Obama’s request.