MOSCOW—Russia, Syria and Iran strongly warned the United States on Friday against launching new strikes on Syria and called for an international investigation of the chemical weapons attack there that killed nearly 90 people.

Russian Foreign Minister Sergey Lavrov, who hosted his Iranian and Syrian counterparts in Moscow, denounced the U.S. missile strikes on Syria as a “flagrant violation” of international law. Additional such actions would entail “grave consequences not only for regional but global security,” Lavrov said.

Syrian Foreign Minister Walid Moallem said the meeting sent a “strong message” to Washington. Iran’s Mohammad Javad Zarif emphasized that the participants agreed that unilateral actions by the U.S. were unacceptable.

The U.S. accuses the Syrian government of deliberately launching the deadly chemical attack in Khan Sheikhoun on April 4. Russia has alleged that the victims were killed by toxic agents from a rebel chemical arsenal hit by Syrian war planes.

Moscow has warned against putting the blame on Damascus until an independent inquiry is conducted and vetoed a proposed U.N. resolution on the attack, saying it failed to mention the need to inspect the affected area.

Lavrov on Friday expressed skepticism about a preliminary investigation conducted by the UN’s chemical weapons watchdog. He alleged that its experts failed to visit the site and said it was unclear to Russia where evidence was taken.

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In Russia’s view, the probe conducted by the Organization for the Prohibition of Chemical Weapons should be widened to include experts from many nations, he said.

“If our U.S. colleagues and some European nations believe that their version is right, they have no reason to fear the creation of such an independent group,” Lavrov added. “The investigation into this high-profile incident must be transparent and leave no doubt that someone is trying to hide something.”

Lavrov said the U.S. strike on the Syrian base has undermined peace efforts in Syria and reflected Washington’s focus on ousting Syrian President Bashar Assad’s government.

“Such attempts won’t succeed,” he said.

The three ministers also discussed the beefing up of U.S. forces on Jordan’s border with Syria, Moallem said. He added that Russia, Iran and Syria have “common procedures against any aggression,” but wouldn’t offer specifics.

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Lavrov said Moscow has asked Washington about the purpose of the buildup and received assurances that the U.S. troops were deployed there to cut supply lines between the Islamic State group factions in Syria and Iraq.

“We will keep monitoring the issue, since the only possible reason for using military force on the territory of Syria is to fight terrorism,” Lavrov said.

Russia has staunchly backed Assad’s government throughout Syria’s six-year civil war. It has conducted an air campaign in Syria since September 2015, saving Assad’s government from imminent collapse and helping to reverse the Syrian military’s fortunes.

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