PENTAGON / WASHINGTON - The United States appears to be raising the stakes in Syria, suggesting Russia may have helped the Assad regime carry out a deadly chemical weapons attack that killed more than 80 people in Idlib province.

A day after firing 59 Tomahawk missiles at Syria’s Shayrat Airfield, the launching point for the chemical weapons strike, senior American military officials said they were looking at evidence that the Syrian regime did not act alone.

US Looks at Russia’s Role in Syria’s Nerve-Gas Attack

“We think we have a good picture of who supported them,” a senior military official told Pentagon reporters, adding officials were “carefully assessing any information that would implicate the Russians” — confirming they either knew of Tuesday’s sarin gas attack in advance or assisted Syrian government forces.

US Looks at Russia’s Role in Syria’s Nerve-Gas Attack

The Pentagon’s comments Friday followed a series of terse criticisms from Washington about Russia’s role in Syria, some accusing Moscow of trying to “sow confusion” over Syria’s use of chemical weapons by promoting what they called “false facts.”

US Looks at Russia’s Role in Syria’s Nerve-Gas Attack

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“Damascus and Moscow assured us all these weapons had been removed and destroyed,” a U.S. official told VOA, referencing an agreement in 2013 to eliminate Syria’s chemical-weapons stockpiles.

“Russia faces a choice,” the official continued. “Either it takes responsibility for ensuring the removal of these weapons, as Russia committed it would do, or it admits that it lacks the ability to control [Syrian President Bashar al-] Assad.”

Pentagon officials refused to offer any direct evidence Friday linking Russia to the April 4 gas attack on town of Khan Sheikhoun, which was hit by bombs containing a chemical consistent with sarin, an extremely potent and deadly nerve-gas agent. However, they noted that a Russia military aviation unit is based at the same airfield, and that Russian forces in Syria are known to have “chemical expertise.”

U.S. military officials said Friday they had watched a small drone flying over a hospital in Khan Sheikoun that was both a target of the chemical attack and also provided treatment for gas victims.

“About five hours later, the UAV [drone] returned, and the hospital was stuck by additional munitions,” one official said.

A senior military official suggested that was an attempt “to hide the evidence of a chemical attack.”

Syria has claimed that its airstrike Tuesday in Khan Sheikhoun was carried out with conventional explosives, which may have inadvertently detonated a stockpile of sarin gas in a warehouse controlled by anti-Assad rebels. That theory was ridiculed by Western experts on Syrian military and other analysts.

Syrian authorities condemned the U.S. missile strike, terming it a “flawed U.S. strategy” that “makes the U.S. a partner of Islamic State and [the al-Nusra Front] and other terrorist organizations.’’

Russia, meanwhile, opened fire in its own war of words with Washington.

Prime Minister Dmitry Medvedev took to social media, branding the U.S. missile strike an illegal act and warning that Washington was now “one step away from military clashes with Russia.”

Moscow also claimed it shut down a direct line of communication with American forces in the area around Syria. The communications link was established soon after Russian military forces arrived to assist Assad, more than a year ago, with the aim of avoiding near collisions or other midair incidents involving Russian and U.S. aircraft operating in the same Syrian airspace.

U.S. officials denied the communications link had been shut down. They confirmed it had been used to warn Russia of the U.S. missile strike in advance, and that it remained operational afterwards.

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Pentagon officials said they took precautions to avoid striking Russian personnel stationed at the Syrian airfield, and added that they specifically avoided hitting chemical-weapons storage facilities in the area. Military experts told VOA that Tomahawk missiles were used in the attack because precision strikes were necessary.

One American military official said the action in Syria early Friday was “appropriate, proportionate, precise and effective.”

Early assessments indicated the U.S. successfully targeted about 20 aircraft, storage facilities, ammunition supply bunkers and radars, yet doubts are beginning to emerge about the missile strike’s effectiveness.