DAMASCUS: A suicide bombing rocked a Damascus mosque on Thursday, killing at least 42 people including Syria's most prominent pro-regime Sunni cleric, and wounding 84 others, state media and a watchdog group said.

The attack came as heavy fighting rumbled on across Syria, and UN Secretary General Ban Ki-Moon said the United Nations would investigate whether chemical weapons have been used in the conflict.

The bomber blew himself up inside the Iman Mosque in central Damascus as the cleric, Mohamed al-Bouti, addressed religious students, in an attack that echoed the sectarian violence in Iraq.

Bouti was the most senior pro-regime Sunni cleric in Syria whose weekly addresses at Friday prayers were frequently broadcast live on state television.

His death will be a blow to the regime of President Bashar al-Assad, which has been fighting an insurgency that flared when his forces launched a bloody crackdown on Arab Spring-inspired protests that erupted in March 2011.

The relentless violence has killed tens of thousands of people and caused more than one million to flee their homes as refugees.

“Senior cleric Dr Mohammed Saeed Ramadan al-Bouti was martyred in a terrorist suicide attack at the Iman Mosque in Mazraa,” a district of the capital, said state television. “Bouti was martyred while he was giving a religious lesson to religion students in the Iman Mosque.”

The official Al-Ikhbariya television station aired gruesome footage from inside the mosque, where dozens of corpses and body parts, including limbs and hands, were strewn on the carpeted floor among pools of blood.

The footage showed emergency workers collecting the remains from inside the mosque and carrying them out in grey body bags.

Syria's health ministry observed that at least 42 people had been killed, including a prominent pro-regime Sunni cleric and his grandson, and 84 others had been wounded.

Born in 1929, Bouti was from a large Kurdish family and spent years studying Islamic theology, including at Cairo's al-Azhar University, according to state television.

He was reviled by the opposition, and frequently lashed out against the rebels seeking the overthrow of Assad's regime, encouraging Syrians to join the national army to fight against the insurgents.

Assad's regime is dominated by members of the minority Alawite sect and had relied in part on the cleric to bolster its claims to represent all Syrians, including Sunnis, from whom the opposition draws much of its support.

The attack came as fighting continued throughout the country, with rebel forces reportedly gaining ground in the Golan Heights, which is partly occupied by Israel.

“It appears that the rebels launched coordinated attacks on multiple parts of the Golan, taking control of areas and villages in the province of Quneitra,” Observatory director Rami Abdel Rahman told AFP.

But state media reported the “armed forces restored security and stability to the towns of Al-Talal al-Hamr and Khan Arnaba (in Quneitra) after eliminating several Al-Nusra Front terrorists.” Lebanese security sources told AFP that around 220 Syrians, including several wounded had crossed the border from southern Syria over the past 24 hours.

A security official in Damascus told AFP that some 2,500 trained and equipped rebels had entered the Daraa region in recent weeks, following reports that American specialists were training rebels in the kingdom.

Louay Moqdad, a spokesman and coordinator for opposition forces, acknowledged that several nations had begun training rebel forces, but declined to provide further details.