MANILA — A grenade was lobbed into a mosque in the southern Philippines before dawn on Wednesday, killing two Muslim religious leaders, in the second attack in days on a place of worship in the restive south, officials said.

The attack, in the city of Zamboanga, occurred three days after a bombing at a cathedral on the nearby island of Jolo, which killed 20 people outright. More than 100 people were wounded in that attack, one of whom died on Tuesday, bringing the death toll to 21.

The police said it was too soon to identify a motive for the Wednesday mosque attack.

The country’s defense secretary, Delfin Lorenzana, said it was “unlikely” that the two bombings were connected.

“We’re still looking at it, but we haven’t seen any connection,” Mr. Lorenzana told reporters, discounting the possibility that the mosque attack was revenge for the cathedral bombing. “The president has said in Jolo that the Christians and Muslims have been coexisting for a hundred years and there have never been revenge attacks.”