At least 20 people are confirmed dead after the 485ft-long Shakidor Dam burst late on Thursday near Pasni, a remote village in Baluchistan province. Between 400 and 500 people are still unaccounted for, said provincial cabinet minister Sher Jan Baluch.
"The army has started rescue operations to try to save as many lives as possible," said Baluch. He said he feared some of the missing may have been swept into the sea, though others might be taking shelter in homes or surrounding areas.
More than 1200 villagers have been pulled alive from the waters by troops, helicopters and boats sent to the area, said Mudasser Butt, an army spokesman in Quetta.
A dozen truck drivers and other bystanders were swept into the Arabian Sea when the dam burst on Thursday.
Pakistan has had more than a week of rain and snow, which in some parts of the country were the heaviest in seven years, said Qamar-uz Zaman Chaudhry, the head of the nation's meteorological department.
More severe weather was expected in the next two days, he said.
Pasni is about 650km south of Quetta, the capital of Baluchistan province.