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WASHINGTON (CNN) – Syria and its most powerful ally, Russia, blamed Israel for striking an air base in the war-torn country on Monday, following a suspected chemical gas attack that drew condemnation from world powers.

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Russia’s Defense Ministry claimed two Israeli F-15 warplanes launched airstrikes on the T-4 base in central Syria from Lebanese territory, the state-run RIA Novosti news agency reported. Syrian state media said that a number of people were killed or injured in the strikes and cited a military source blaming Israel for the military action.

At least four Iranian military advisers were killed, Iranian state media news agency FARS reported.

Israeli officials have not issued any response to reports of the strike, but its warplanes hit the same facility in February after Israel said an Iranian drone had infiltrated its airspace. Israeli officials said Iran, a key backer of the Syrian government, was using the T-4 facility as a command center.

The Syrian conflict has developed into a complex proxy war, which in recent years has included Israeli strikes on Iran-linked targets.

Israel’s official stance on the war is neutral, but it has acknowledged carrying out some strikes on Syrian military targets while denying responsibility for numerous others. It has clearly stated in the past that it would respond to Iran’s growing presence in Syria.

Defense Minister Avigdor Lieberman said in a March interview with the Yedioth Ahronoth newspaper that Israel had been acting militarily in Syria in recent months. “A large part of Israeli army activity does not find its way on to (Israeli newspapers and websites) … and that is good,” he said. “It’s just not right to say that we don’t act.”

Israel and Russia have coordinated their military actions over Syria for deconfliction since Russian forces entered the country in late 2015. The T-4 base, located in a strategic position between the cities of Homs and Palmyra in Syria’s west, has been a key staging ground for Syrian and Russian aircraft during the war.

Syria claimed that its air defenses had shot down eight missiles fired at the air base Monday, while RIA Novosti reported three missiles flew to their target and a remaining five were shot down.

The country had earlier speculated that the US was behind Monday’s strikes, which the Pentagon swiftly denied.

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Syrian activist groups said toxic gas inside barrel bombs were dropped from helicopters over the rebel-held city of Douma on Saturday, killing dozens of civilians and wounded scores more.

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“We received many patients who suffered from symptoms compatible with exposure to chlorine gas, high concentration chlorine gas,” said Dr. Ahmad Tarakji with the Syrian-American Medical Society.

Graphic footage shot by rescuers and activists show victims — including children — dead and injured, some ghostly white and foaming at the mouth in makeshift clinics. Others were found suffocated in their homes, according to first responders.

The Organisation for the Prevention of Chemical Weapons said Monday that a fact-finding mission was working to establish whether chemical weapons were used.

US President Donald Trump on Sunday warned Russia, Iran, and Syria of a “big price to pay” following the attack, and slammed Syrian President Bashar al-Assad as an “animal” on Twitter.

On Monday, Trump said the administration is considering how the U.S. should respond and all options are reportedly on the table.

“We are meeting with our military and everybody else. We’ll be making some major decisions over the next 24-48 hours,” he said.

The Syrian government and Russia vehemently denied involvement and accused rebels in Douma of fabricating the chemical attack claims in order to hinder the army’s advances and provoke international military intervention.

Russian Foreign Minister Sergey Lavrov said Monday that his country had sent experts to Douma and that there was “no trace” of the use of chemical weapons there.

The attack came as Syrian forces were on the verge of reclaiming Douma, the last town held by rebels in Eastern Ghouta, which was besieged for six years and has been heavily bombarded since mid-February.

On Sunday, Syrian state TV reported that the government had reached an agreement with Jaish al-Islam, the only remaining rebel group in the town, to leave in the next 48 hours.

As part of the agreement, the group’s fighters would be transported to Jarablus in northern Syria. In exchange, the rebels would release all captives they are holding in Douma.

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