Suspected government warplanes carried out several airstrikes in Syria's Hama on Thursday, killing at least 25 people, amid a lightning advance by insurgents on government-controlled areas of the central province.

The rebel offensive is led by an ultraconservative Islamic group, Jund al-Aqsa, and several factions from the western-backed Free Syrian Army. In the past three days, the insurgents have pushed their way from northern Hama, where they are usually based, down south toward government-held areas.

Local activist Ahmed al-Ahmed said the rebels were only eight kilometers (five miles) away from the provincial capital, Hama. The insurgents have taken over a government military base and are in control of several towns along the highway linking it to the capital, Damascus, following a "surprising" government retreat, he said.

The advances in Hama are significant because if rebels control the city and the highway they can sever government supply lines and deprive President Bashar Assad of a traditional stronghold. Clashes are now concentrated around a hill outside the provincial capital, also called Hama, al-Ahmed said.

Al-Ahmed, who is currently in Turkey, said government forces in Hama province may have been weakened because many troops were transferred to Aleppo city, where they are bogged down in vicious fighting with advancing rebels.

The Britain-based Syrian Observatory for Human Rights confirmed the insurgents' advance and said a series of airstrikes since early Thursday had killed at least 25 civilians, including six children, in northern Hama province.

The Hama-based Syrian Press Center, an activist group operated by al-Ahmed, said at least ten people were killed when warplanes struck a crowd of people displaced from Suran, a town north of Hama city that was seized by opposition fighters. Another 15 people were killed further to the west, the center said.

Syria's state news agency, SANA, says warplanes killed 10 "terrorists" in northern Hama.