The US military has concluded that its largest conventional bomb is not capable of destroying Iran's most heavily fortified underground facilities suspected to be used for building nuclear weapons, The Wall Street Journal reported late Friday.

But citing unnamed US officials, the newspaper said the military was stepping up efforts to make it more powerful.

The 13.6-ton "bunker-buster" bomb, known as the Massive Ordnance Penetrator, was specifically designed to take out the hardened fortifications built by Iran and North Korea, the report said.

But initial tests indicated that the bomb, as currently configured, would not be capable of destroying some of Iran's facilities, either because of their depth or because Tehran has added new fortifications to protect them, the paper noted.

In a report issued in November, the International Atomic Energy Agency said intelligence from more than 10 countries and its own sources "indicates that Iran has carried out activities relevant to the development of a nuclear device."

It detailed 12 suspicious areas such as testing explosives in a steel container at a military base and studies on Shahab-3 ballistic missile warheads that the IAEA said were "highly relevant to a nuclear weapon programme."

Iran, which has come under unprecedented international pressure since the publication of the report, with Washington and the EU targeting its oil sector and central bank, rejected the dossier as based on forgeries.

Meanwhile, doubts about its bomb'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 Journal noted.

The Defense Department has spent about $330 million so far to develop about 20 of the bombs, which are built by Boeing Co., the report pointed out.

The Pentagon is seeking about $82 million more to make the bomb more effective, The Journal said.

Defense Secretary Leon Panetta, in an interview with The Journal Thursday, acknowledged the bomb's shortcomings against some of Iran's deepest bunkers.

He 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 said.