Indian rescuers are searching for 20 students and a professor swept away by waters released from a dam while the group was bathing in a river in the mountainous northern state of Himachal Pradesh.

Rescuers found four bodies and officials say prospects of finding the rest alive are bleak after the students, who were on a college trip from southern India, were swept away when the waters of the river Beas spiked dangerously high on Sunday.

"I think they were playing in the water," said a police official, who declined to be named because he was not authorized to speak to the media. "The water rose and they were washed away."

Whether warning signals were sounded before the dam released water is a matter of dispute.

"There was no warning," said D. Naidu, principal of the VNR Vignana Jyothi Institute of Engineering and Technology, the south Indian college from which 65 students had set off on the trip.

"They were trying to take photos on the banks when they were washed away," he told Reuters, saying he spoke to the survivors.

Others disputed the account.

Mandeep Singh, an engineer on the dam's power plant, told news channel CNN-IBN that staff sounded warnings before releasing the water and local residents "definitely warned the students to come out of the river" several times.

Officials have launched an investigation to determine the sequence of events, said police official Rajesh Kumar, but are now focused on rescue operations to find the rest of the group.

"The chances are minimal" to find them alive, he said, "but we're trying."