MOSCOW—The Kremlin Friday dismissed as unconvincing evidence that U.S. officials provided of Syrian leader Bashar al-Assad’s alleged use of chemical weapons and criticized Washington’s decision to arm Syrian opposition fighters, but stopped short of threatening to deliver air-defense missiles to the Assad government in response.

A senior Kremlin official said Moscow is “not yet” discussing the delivery of the advanced air-defense system in the wake of the U.S. decision. Last month, Russian officials threatened to fulfill the 2010 contract for the S-300 missiles as a way to deter potential outside military intervention in the two-year-old Syrian civil war. Western powers and Israel have staunchly opposed the sale of the system.

Both Moscow and the U.S. are pushing the warring sides in Syria to enter peace talks in the coming months. But opposition forces have appealed for more weapons and support in recent weeks as they’ve lost ground against Mr. Assad’s troops and their allies from the Iranian-backed Hezbollah group. The Kremlin opposes any international action against its longtime client, Mr. Assad, and has been skeptical of past Western claims that his forces were using chemical weapons.

Thursday, President Barack Obamaauthorized the U.S. administration to arm fighters against the Assad regime, reversing a long-running policy of giving only nonlethal support to the country’s opposition. The White House cited confirmation that Mr. Assad’s regime had killed up to 150 people with chemical weapons as the reason for its about-face.

Russian officials say the evidence isn’t rock solid. “I want to confirm that we had a meeting with American representatives in which Americans tried to present information to us about the regime’s use of chemical weapons, but frankly speaking, the evidence Americans set out looks unconvincing,” Yuri Ushakov, the Kremlin’s top foreign-policy aide, said Friday, according to Russian news agencies.

Mr. Ushakov cited the flawed intelligence assessment from the administration of former President George W. Bush about weapons of mass destruction in the run-up to the Iraq war but said he didn’t want to “draw any parallels.”

Other Russian officials were more direct. “The data on Assad’s use of chemical weapons is fabricated just like the lies about weapons of [Saddam] Hussein’s weapons of mass destruction,” read a tweet on the feed of Alexei Pushkov, chairman of the Russian parliament’s international relations committee. “Obama is going down the path of G. Bush.”

Ben Rhodes, the White House Deputy National Security Advisor for Strategic Communications, said the conclusion that Mr. Assad’s regime has wielded chemical weapons was based on physical samples taken from Syria. He said the U.S. relied on “multiple, independent streams of information” and has “high confidence” in the assessment. He cited four dates and locations during which the U.S. thinks Mr. Assad’s regime employed chemical weapons.

Mr. Ushakov, who long served as the Russian ambassador to the U.S., said any extension of the White House’s support for Syria’s opposition fighters won’t help a joint effort by the U.S. and Russia to bring the bloody conflict’s opposing sides to the negotiating table.

Russian Ministry of Foreign Affairs spokesman Alexander Lukashevich said the move suggests U.S. efforts to bring the opposition to the peace talks had stalled. He said pumping arms into Syria will increase “the level of armed confrontation and violence against civilians” and reiterated Russia’s commitment to a peaceful resolution to the conflict.

The U.S.’s decision to arm Syria’s opposition fighters could change the balance of power on the ground in the war-torn country, where Mr. Assad’s troops recently have been gaining an edge over beleaguered opposition fighters. U.S. officials have stressed that Mr. Assad, backed by Hezbollah, has been making gains more than two years into a violent conflict that has left more than 90,000 people dead, according to United Nations figures.

ReutersA Free Syrian Army fighter wearing a gas mask carries his weapons as he walks past a damaged tank near Idlib, Syria.

The U.S. move comes ahead of a high-stakes meeting between Mr. Obama and Russian President Vladimir Putin on the sidelines of the G-8 summit in Northern Ireland that starts on June 17. Syria will be at the top of the agenda for the sidelines meeting.

The Syrian regime is a longtime Kremlin ally dating back to the days of the Cold War. Mr. Putin has opposed outside military intervention in the Syrian conflict, and Russia has joined China in vetoing three UN resolutions that were aimed at forcing Mr. Assad to step down.

Mr. Ushakov stressed that the U.S. and Russia aren’t “competing on Syria,” and are trying to find a constructive way to solve the problem in the region.

Syria’s government signed a contract in 2010 to buy four S-300 batteries with 144 missiles for $900 million, and the first deliveries were scheduled to start in summer 2013. The weapons could change the power dynamic in the Middle East and help the Assad regime prevent the sort of military campaign Western governments organized to aid rebels fighting Col. Moammar Gadhafi in Libya.