Underground explosions are usually a safe option for testing nuclear devices and North Korea's test yesterday had apparently not caused any radioactive contamination, Indian atomic scientists said.

India and its neighbour and old rival Pakistan were the last countries to carry out nuclear tests when they conducted tit-for-tat explosions in May 1998.

The tests drew global condemnation and sanctions were imposed on both countries but the safety of the tests was never questioned, said the scientists who were closely involved with the programme.

Geological conditions at the test site and the design of the tunnels are crucial to preventing contamination from an explosion and initial reports indicated that no radioactivity had been monitored hours after Pyongyang's test, they said.

"Neighbours like Japan, (South) Korea and Taiwan all have excellent atmospheric radiation monitoring capability," said A Gopalakrishnan, former chief of India's Atomic Energy Regulatory Board.