NEW YORK - The United States has intensified efforts for a more powerful bomb after acknowledging that the its largest conventional bomb is not capable of destroying Iran's most heavily fortified underground nuclear facilities, according to a media report.The 13.6-ton "bunker-buster" bomb, known as the Massive Ordnance Penetrator (MOP), was specifically designed to take out the hardened fortifications built by Iran and North Korea, The Wall Street Journal said in a report on Monday.In an interview with the Journal, Defence Secretary Leon Panetta, while acknowledging, the bomb's shortcomings against some of Iran's deepest bunkers, said more development work would be done and that he expected the bomb to be ready to take on the deepest bunkers soon. "We're still trying to develop them," Panetta was quoted as saying.The efforts for boosted "bunker busting" capability have been speeded up as part of contingency planning for a possible strike against Iranian nuclear sites, the newspaper 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 MOP's effectiveness prompted the Pentagon this month to secretly submit a request to Congress for funding to enhance the bomb's ability to penetrate deeper into rock, concrete and steel before exploding, the officials said.Within the president's new defence budget plan is funding for an intriguing new item: a floating drone base that also could be used as a launching pad for commandos.The Defence Department has spent about $330 million so far to develop about 20 of the bombs, which are built by Boeing Co. The Pentagon is seeking about $82 million more to make the bomb more effective, according to government officials briefed on the plan.Some experts question if any kind of conventional explosives are capable of reaching facilities such as those built deep underground in Iran. But US defense officials say they believe the MOP could already do damage sufficient to set back the programme.President Barack Obama has made clear that he believes US and international sanctions can curb Iran's nuclear program if they are given more time to work. At the same time, however, Obama has asked the Pentagon to come up with military options.In Tuesday's State of the Union address, Obama said: "Let there be no doubt: America is determined to prevent Iran from getting a nuclear weapon, and I will take no options off the table to achieve that goal." Iran denies it is trying to develop atomic weapons.The US has sought in recent weeks to tamp down tensions with Iran, but the Pentagon is at the same time pushing ahead with contingency planning."The development of this weapon is not intended to send a signal to any one particular country," Pentagon press secretary George Little said. "It's a capability we believe we need in our arsenal and will continue to invest in it."Officials said the planned improvements to the MOP were meant to overcome shortcomings that emerged in initial testing. They said the new money was meant to ensure the weapon would be more effective against the deepest bunkers, including Iran's Fordow enrichment plant facility, which is buried in a mountain complex surrounded by antiaircraft batteries, making it a particularly difficult target even for the most powerful weapons available to the USDeveloping an effective bunker-buster is complicated in part because of the variables, experts say. Penetration varies depending on factors such as soil density and the types of stone and rock shielding the target.Boeing received a contract in 2009 to fit the weapon on the US's B-2 Stealth Bomber. The Air Force began receiving the first of the bombs in September, a time of growing tensions with Iran. The Air Force has so far contracted to buy 20 of the bombs, and more deliveries are expected in 2013, after additional tests are made.Should a decision be made to use the MOP as currently configured, it could cause "a lot of damage" to Iran's underground nuclear facilities but wouldn't necessarily destroy them outright, Panetta said."We're developing it. I think we're pretty close, let's put it that way. But we're still working at it because these things are not easy to be able to make sure that they will do what we want them to."Panetta added: "But I'm confident, frankly, that we're going to have that capability and have it soon,"The decision to ask now for more money to develop the weapon was directly related to efforts by the US military's Central Command to prepare military options against Iran as quickly as possible, according to a person briefed on the request for additional funds.A senior defence official said the US had other options besides the MOP to set back Iran's nuclear program. "The Massive Ordnance Penetrators are by no means the only capability at our disposal to deal with potential nuclear threats in Iran," the official said.Another senior US official said the Pentagon could make up for the MOPs' shortcomings by dropping them along with other guided bombs on top of a bunker's entry and exit points—provided the intelligence is available about where they are all located.Successful strikes on bunker entry and exit points could prevent an enemy from accessing such a site and could cause enough damage to stop or slow enrichment activity there."There is a virtue to deepness but you still need to get in and out," the senior US official said.The Pentagon was particularly concerned about its ability to destroy bunkers built under mountains, such as Iran's Fordow site near the Shiite Muslim holy city of Qom, according to a former senior US official who is an expert on Iran.The official said some Pentagon war planners believe conventional bombs won't be effective against Fordow and that a tactical nuclear weapon may be the only military option if the goal is to destroy the facility. "Once things go into the mountain, then really you have to have something that takes the mountain off," the official said.The official said the MOP may be more effective against Iran's main enrichment plant at Natanz but added: "But even that is guesswork."The Pentagon notified Congress in mid-January that it wants to divert around $82 million to refine the MOP, taking the money from other defense programs. The decision to sidestep the normal budget request process suggests the Pentagon deems the MOP upgrades to be a matter of some urgency.Panetta said Iran wasn't the only potential target. "It's not just aimed at Iran. Frankly, it's aimed at any enemy that decides to locate in some kind of impenetrable location. The goal here is to be able to get at any enemy, anywhere," he saidPanetta and former Defense Secretary Robert Gates have argued that a military strike would at best delay Iran's nuclear development for a few years. Advocates of a strike say such a delay could be decisive by buying time for other efforts to thwart the program.According to Air Force officials, the 20.5 foot-long MOP carries over 5,300 pounds of explosive material. It is designed to penetrate up to 200 feet underground before exploding. The mountain above the Iranian enrichment site at Fordow is estimated to be at least 200 feet tall.Israel has large bunker-buster bombs but the US hasn't provided the MOP to any other country.