Washington: The US military has stepped up efforts to make their largest conventional weapon, the 13.6 tonne "bunker-buster" bomb, more powerful and capable of destroying Iran's most heavily fortified underground nuclear facilities.
The efforts have been speeded up as part of contingency planning for a possible strike against Iranian nuclear sites, the Wall Street Journal reported quoting US officials.
The move comes after Pentagon war planners concluded that their largest bomb isn't yet capable of reaching Tehran's nuclear weapon-making facilities buried deep underground.
The "bunker-buster" bomb, known as the Massive Ordnance Penetrator, was specifically designed to take out hardened fortifications built by Iran and North Korea to cloak their nuclear programme, the paper said quoting US officials.
But, early tests indicated that the bomb as currently primed was not fully capable of destroying some of Iran's facilities, because Tehran had added new fortifications to protect them.
Doubts about the bomb's effectiveness, US officials said had prompted Pentagon this month to secretly approach the Congress for funding to enhance the "bunker-buster's" ability to penetrate deeper into rock, concrete and steel before exploding.
The US Defence Department has spent $330 million so far to develop about 20 such bombs. The Pentagon is seeking $82 million more to make them effective, the paper said quoting government officials briefed on the plan.
The US aviation giant Boeing received a contract in 2009 to fit the weapon on the American Air Force B-2 Stealth bomber.
Defence Secretary Leon Panetta in an interview to the paper acknowledged the bomb's shortcomings against some of Iran's deepest bunkers and said development work was being done and he expected a more powerful "bunker-buster" to be ready soon.
The Pentagon officials said the new money was meant to ensure that the weapon would be more effective against facilities such as Iran's Fordo enrichment plant, which is buried in a mountain complex surrounded by anti-aircraft batteries near Qom city, making it a particularly difficult target even for the most powerful weapons in the US armoury.
According to US Air Force officials, the 20 foot-long "bunker-buster" carries 5,300 pounds of explosive material and is designed to penetrate upto 200 ft underground before exploding.
The mountain above Iran's enrichment site at Fordo is estimated to be at least 212 feet tall.
The WSJ said that Israel is the only country besides the US to possess such bombs.