The Syrian army is sending reinforcements to face a major rebel offensive in Hama province, a Syrian military source told Reuters on Wednesday, as insurgents pressed an attack in a western area critically important to President Bashar al-Assad.

The assault spearheaded by jihadist fighters got under way on Tuesday and follows two attacks on Assad's seat of power in Damascus in recent days, showing the lingering threat posed by rebels even as Assad enjoys the military upper hand in the war.

The areas of Hama province targeted in the latest assault form part of the western region of Syria where Assad has shored up his rule during the six-year-long war with crucial military support from Russia and Iran.

The Syrian Observatory for Human Rights, a UK-based organization that monitors the war, said insurgents had captured the towns of Soran, some 20 km north of Hama city, and Khattab, some 10 km northwest of it, confirming rebel accounts.

The military source said there were clashes in both places but the situation was not yet decided, adding that the insurgents had mobilized large numbers for the assault.

"Reinforcements are now being sent" by the army, the source said. "There are fierce battles between the two sides."

The attack is being led by Tahrir al-Sham, an alliance of Islamist factions dominated by a group that was formerly al Qaeda's official affiliate in the Syrian war. Groups fighting under the Free Syrian Army banner are also taking part.

An FSA rebel commander, in an interview with the pro-opposition Orient TV, said the Hama assault had been long in the planning, and insurgents were ready for a protracted fight.

"The battle - praise God - has been prepared for a long time and all capacities have been prepared for it so that we can wage a long battle," said the FSA commander identified as a lieutenant in the Ezza Army group.

The escalation in Hama province follows two rebel assaults on government-held areas in Jobar in Damascus, launched by opposition fighters from the Eastern Ghouta, an opposition stronghold east of the capital.

State media said on Wednesday the army was waging fierce clashes with insurgents in northern Jobar.

The Ezza Army commander said the rebel attack in Hama was coordinated with the one in Damascus. He also said that more attacks would be launched "in a number of other areas".