NATO called it one of the largest deployments from Russia in decades. Retired US Lt. Gen. Mark Hertling, a CNN military analyst, said the frigate appeared to be a show of force by Russia, which since late 2015 has been conducting airstrikes in Syria against forces opposing the Syrian regime.

"I think the Russians were caught off guard (by the US strikes)," Hertling said. "So they want to make sure they're tracking those (US) ships."

An airstrike killed 15 civilians, including four children, in Syria's northwestern province of Idlib on Saturday, the Syrian Observatory for Human Rights monitor said. The monitoring group said the strike on the village of Urum al-Joz was suspected to have been carried out by Russian planes, which operate in support of the Syrian government. Syria's Civil Defence volunteer group, also known as The White Helmets, said the death toll was 16, with 20 people injured.

A US-led coalition also has long been conducting airstrikes in Syria, against ISIS targets in the country. On Saturday, a suspected coalition strike killed at least 15 civilians in north-central Syria near Raqqa, the de facto capital of ISIS, a local activist group and the state-run SANA news outlet said.