Thirteen people died when a wall collapsed on Wednesday, while five people, including two women, were killed elsewhere in the city.

"According to initial reports, the deceased were drug addicts who had taken shelter in the building due to the rain," the Rescue 1122 emergency service said of the 13 victims.

Light to moderate rain fell across Sindh province, of which Karachi is the capital, but was heavier in some areas, meteorologists said.

Rain in March is common in Punjab province in east-central Pakistan but unusual in Sindh, which is in the southeast.

"This kind of extreme weather event hasn't happened for a long time," Pakistan Meteorological Department's senior official Ameer Hyder Laghari told AFP.

Pakistan, where 45 percent of people live below the poverty line, is among the countries most vulnerable to climate change, with limited resources dedicated to adaptation.

While South Asia's seasonal monsoon brings rainfall that farmers depend on, climate change is making the phenomenon more erratic.

Last year, monsoon rains in Pakistan killed more than 1,000 people.