A letter from a previously unknown group outlining the threatened attacks in Bangalore was faxed to several newspapers late on Thursday, the director-general of police, BS Sial, said.

"It will be the most coordinated attack the country has ever seen," television channel CNN-IBN quoted the letter, written in English, as saying.

Six attackers will trigger explosions including, "two human bombs to target the state chief minister," Moin-ud-Din of the Al-Jehadi group said, according to the report.

"Newspapers have received the letter late last night, and we are trying to find out if it is a hoax. But we will not take any chances," said Sial, without giving further details.