Flooding caused by monsoon rain washed out a railway bridge in Pakistan and cut the line to the southern financial hub of Karachi, officials said Monday.

A river rose by about 20 feet (six metres) and swept away part of the bridge across a gully at Ran Pathani, 80 kilometers (48 miles) north of Karachi, railways official Syed Nasir Zaidi told AFP.

It also damaged the pillars of the bridge and carried off some of the tracks.

All ten daily trains serving Karachi are now stopping at Hyderabad city, 120 kilometres to the northeast, with buses covering the rest of the route.

"We are waiting till the water recedes and I assess that the repair work could be started tomorrow," Zaidi said.

Torrential rain in Pakistan has killed at least 30 people in the past two weeks and caused widespread damage to crops, roads and triggered landslides.

In neighbouring India, the monsoon death toll has exceeded 300 according to one count, although officials are also predicting the bountiful rains will yield bumper crops.