Two express trains derailed into a river in central India overnight after flood water weakened the tracks, killing 21 people, state officials said on Wednesday, reigniting criticism about lack of safety on the world's fourth-largest rail network.

Divers using gas cutters pulled out trapped passengers and 300 had been rescued by early morning, officials said. Dozens were rushed to hospital.

Twelve coaches of the Kamayani Express to Varanasi from Mumbai derailed near Harda in the central state of Madhya Pradesh just before midnight. Six coaches ofthe Janata Express derailed around the same time, the railway ministry said.

"The prima facie cause of the incident is stated to be flash floods due to heavy rains," Railway Minister Suresh Prabhu told parliament.

Heavy monsoon rains and the tail-end of a cyclone have killed more than 100 people across India in flooding, landslides and building collapses in recent days.

But a former railway minister said if the tracks had been maintained, the accidents could have been avoided.

"It's totally unacceptable. It's a symptom of a deep-rooted cancer in the railway system," Dinesh Trivedi told reporters.

Train wrecks killed more than 25,000 people on India's overloaded and outdated railway network last year alone.

Prime Minister Narendra Modi this year massively increased spending to modernize the network, but improvements will take years.

The head of the railway board, AK Mittal, told reporters a flash flood struck the area just minutes before the trains came through, disturbing the tracks. "Ten minutes back at this spot, train movement was normal. There was no problem."

Television images showed train coaches smashed into each other and tilted into the river. Some police and villagers had clambered on top of the coaches lying on their side.

"It shook like a tree in storm. We thought something had hit the train," said Santosh Singh, a passenger.

India's state-run railway network, the fourth largest after the United States, China and Russia, carries some 23 million passengers each day.