Russia has warned the U.S. against repeating last week's strike in Syria, an action it called a "flagrant violation" of international law, and called for an expansion of a probe into the chemical weapons attack that prompted American action.

"We have reiterated our position and were united in stating that the attack was an act of aggression, which blatantly violated the principles of international law and the U.N. charter," Russian Foreign Minister Sergey Lavrov said Friday at a meeting with his Syrian and Iranian counterparts in Moscow.

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"We call on the U.S. and its allies to respect Syria's sovereignty and refrain from actions similar to what happened on April 7 and which have serious ramification not only for regional but also global security," he added.

Syria's foreign minister, Walid Moallem, and Iran's foreign minister, Mohammad Javad Zarif, attending the meeting with Lavrov, concurred.

Syrian President Bashar Assad has denied responsibility for the April 4 chemical weapons attack that left more than 80 dead, and his allies in Russia have instead blamed rebels for the strike, claiming the poisonous agent was dispersed after conventional weapons landed on a rebel weapons depot.

The U.S. has concluded that Syria is responsible for the attack, using it as justification to launch the strike against a nearby Syrian government airfield.

U.S. officials this week circulated satellite images, reports from the scene and details of exposure gathered from victims that they say makes them "confident that the Syrian regime conducted a chemical weapons attack, using the nerve agent sarin, against its own people."

Kremlin spokesman Dmitry Peskov, traveling with President Vladimir Putin in Kyrgyzstan, told reporters Friday that the U.S. had failed to provide conclusive evidence.

The Organization for the Prohibition of Chemical Weapons is investigating the attack. In a statement, the United Nations Commission of Inquiry on Syria warned that the use of chemical weapons and what is believed to be the deliberate subsequent bombing of a medical facility where victims of the gas attack were being treated "would amount to war crimes and serious violations of human rights law."

On Wednesday, Moscow vetoed a draft U.N. resolution condemning Assad's forces for the attack and has called for the investigation to involve experts from many nations.