At least 13 people were killed and more than 20 are still missing after two passenger boats collided in the sea off Thatta, officials said Thursday.

The boats, carrying more than 60 people in total, were ferrying passengers to visit a popular Sufi Muslim shrine in Sindh when they collided and overturned in heavy winds near the ancient city of Thatta, district official Mirza Nasir Baig said.

The 13 killed included women and children, and 20 others were pulled from the sea alive, he said.

Provincial officials called for rescuers to rush to find any other survivors before nightfall.

Hundreds of shrines - many that are centuries old - dot Pakistan, where several million Muslims are believed to still follow Sufism, a mystical branch of Islam.

Visiting shrines to pray and offer alms to the poor - and cash to the custodians - is very popular in Pakistan, where many believe it will help get their prayers answered