The United States announced sanctions Thursday against 18 senior Syrian military officers and officials over the use of chemical weapons.

"We condemn in the strongest possible terms the Syrian regime’s use of chemical weapons," said National Security Council spokesman Ned Price.

A UN-backed investigation found that several units of the Syrian military had used toxic weapons against three villages in northern Syria in 2014 and 2015.

Government helicopters flying from two government-controlled air bases dropped chlorine barrel-bombs on the villages of Qmenas, Talmenes and Sarmin, the panel's latest report said.

Chlorine use as a weapon is banned under the Chemical Weapons Convention, which Syria joined in 2013 under pressure from Russia.

In response to its use, the US has targeted individuals including top intelligence officials and military commanders.