The US had notified Russia before launching the Tomahawk missiles on a Syrian army airbase, a Pentagon official said.
"Russian forces were notified in advance of the strike using the established deconfliction line," Xinhua news agency quoted Pentagon spokesman Jeff Davis as saying.
"US military planners took precautions to minimise risk to Russian or Syrian personnel located at the airfield," he said, hinting a Russian military presence at the Shayrat base which was struck by 59 Tomahawk missiles on Thursday night.
US Secretary of State Rex Tillerson blamed Russia for being unable to prevent a chemical weapons attack, saying the US strike was in response to the brutal attack that killed dozens of civilians in Idlib province on Tuesday.
"Either Russia's been complicit or Russia's been incompetent in its ability to deliver" removal of chemical weapons, Tillerson said.
The Pentagon said the strike was intended to deter the Syrian government from using chemical weapons again.
Syrian Foreign Minister Walid al-Moallem said the country's army struck an arms depot containing chemical materials, denying the air force fired toxic gas.