Warplanes pounded rebel-held areas north of the Syrian city of Hama on Thursday in an escalation of air strikes, a rebel official and a monitor said, as government forces fought to reverse the insurgents' biggest assault in months.

The Syrian Observatory for Human Rights and a doctor said air strikes near the town of Latamneh, northwest of Hama, had led several people to choke, saying it was a sign of a gas attack. A Syrian military source denied the army had used any such weapons, branding the reports as rebel propaganda.

Rebel groups, spearheaded by jihadist insurgents from the former al Qaeda-linked Nusra Front but also including Free Syrian Army (FSA) groups, launched their attack on government-held areas near Hama last week.

The area is of vital importance to President Bashar al-Assad, who with the support of Russia and Iran has the military upper hand there.

The observatory, a Britain-based war monitoring group, said jets had hit several rebel towns on Thursday, including Soran and Khattab, both captured by insurgents at the start of the offensive.

The Syrian military source said the army was conducting attack operations in the area and had seized the initiative back from the rebels.