Divers opened the main hatch on Wednesday and were seeking to secure two water-tight compartments before pumping out water to raise the diesel-powered vessel.

Prime Minister Manmohan Singh, addressing the nation in the traditional Independence Day speech at the Red Fort monument in Delhi, lamented the tragedy which represents a major setback for the rapidly modernising navy.

"We have deep regret about the accident involving INS Sindhurakshak in which 18 of our brave men are feared to have been martyred," he said.

"The accident is all the more painful because the navy had recently achieved two major successes in the form of its first nuclear submarine, INS Arihant and the aircraft carrier, INS Vikrant," Singh said.

In recent days India launched its first domestically produced aircraft carrier and began sea trials for the first Indian-made nuclear submarine, trumpeted as a "giant stride" for the country.

The world's biggest democracy has been expanding its armed forces to upgrade its mostly Soviet-era weaponry and respond to what many in India perceive as a growing threat from regional rival China.

But the disaster -- considered to be the Indian navy's worst -- has cast a long shadow over the military's capabilities.

Defence Minister A.K. Antony described the explosion as the "greatest tragedy in recent time".

Navy chief D.K. Joshi said Wednesday that no sign of life had been detected on board since the submarine was engulfed in a fireball.http://www.geo.tv/article-113677-India-works-to-salvage-sub-as-PM-laments-deadly-accident