Syrian media say a suicide bomber has killed at least 42 people in an ancient Damascus mosque, including leading pro-government cleric Mohammed al-Buti.
The media reports say more than 80 other people have been wounded. No one has claimed responsibility for Thursday's blast. But opposition chief Ahmed Moaz al-Khatib tells the French News Agency that killing al-Buti is a crime he completely rejects.
Al-Buti was a longtime supporter of the Assad family and a fierce critic of rebels fighting to topple President Bashar al-Assad.
Earlier Thursday, U.N. Secretary-General Ban Ki-moon ordered an investigation into the Syrian government's allegations that rebels used chemical weapons in a rocket attack. Mr. Ban called on all parties in Syria to grant full access and cooperation to the investigators.
"There is much work to do and this will not happen overnight. It is obviously a difficult mission. I intend for this investigation to start as soon as practically possible. Again, my announcement should serve as an unequivocal reminder that the use of chemical weapons is a crime against humanity," Ban said.
Rebels and the Syrian government accuse each other of using chemical weapons during an attack on a town near Allepo Tuesday.
U.S. President Barack Obama said he is "deeply skeptical" of the claim that rebels used chemical weapons. He said Washington will carry out its own investigation.