NEW DELHI (AFP) - India's home minister called the death of the main suspect in the New Delhi gang-rape case a "major lapse", which he promised would be investigated.

"It is a major lapse in security, certainly it is not a small incident. Action will be taken," Home Minister Sushilkumar Shinde told a press conference in the capital on Monday.

Only an enquiry would establish the cause of Singh's death in New Delhi's maximum-security Tihar prison, he added.

"It is a suicide, the preliminary reports show... but we are awaiting the enquiry report to come," he said. He also said he would instruct the New Delhi city administration to boost security for the four other suspects facing trial in the Dec 16 gang-rape of the 23-year-old student in their bus in New Delhi.

"Tihar is within the jurisdiction of the Delhi (city) government, and so I will give the necessary instructions," Mr Shinde said in reply to questions over the safety of the remaining suspects.

Tihar officials said bus driver Singh had hanged himself with a makeshift noose.