PESHAWAR, Pakistan — Flash floods triggered by monsoon rains swept away a bus carrying a wedding party Saturday in northwest Pakistan, killing 21 people, an official said.

The spokesman for the local disaster management authority, Latifur Rehman, said the incident took place in the Landi Kotal tribal region bordering Afghanistan. At least four people were missing and it was unclear whether the bride and groom were also traveling in the same bus.

Rehman said eight women were among those killed in the incident.

Pakistan’s National Disaster Management Authority said Saturday that as many as 55 people have been killed across Pakistan since the monsoon rains started in July.

Flash floods commonly occur during South Asia’s summer monsoon season, which ends in September.