Air strikes killed at least 18 people and wounded dozens in the northwestern Syrian province of Idlib on Tuesday, the Syrian Observatory for Human rights said.

The attack caused many people to choke or faint, and some had foam coming out of their mouths, the Observatory said, citing medical sources who described it as a sign of a gas attack. Damascus has repeatedly denied using any such weapons.

Syrian government or Russian jets pounded the town of Khan Sheikhoun in rebel-held Idib in the morning, the Britain-based war monitoring group said. The Syrian army could not immediately be reached for comment.

Pro-opposition Orient News television said the strikes had killed 50 people and injured more than 150.