The Britain-based Syrian Observatory for Human Rights says the attacks began on Tuesday morning and that government forces called in Syria's air force to repel them.

The Ibaa media outlet of the al-Qaida-linked Hayat Tahrir al-Sham militant group said its fighters are attacking Syrian positions east of Khan Sheikhoun, a major town that was held by rebels until they lost it last week.

Syrian government forces captured wide areas from insurgents over the past weeks in an offensive that began on April 30.

Taher al-Omar, a citizen journalist with the al-Qaida-linked militants, says they have carried out several suicide attacks so far.