The Pentagon has stepped up efforts to make a bomb capable of destroying Iran's most heavily fortified underground facilities, the Wall Street Journal reported Saturday.

"The 30,000-pound (around 13,600 kg) '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 to cloak their nuclear programmes," the daily said quoting unnamed US officials briefed on the plan.

Initial tests, however, 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, RIA Novosti said.

US Defence Secretary Leon Panetta, in an interview with The Wall Street Journal, 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.

Unnamed US officials said they would ensure the weapon would be more effective against the deepest bunkers, including Iran's Fordow enrichment plant facility.

Fordow is buried in a mountain complex in Iran surrounded by anti-aircraft weapons, which makes it a very difficult target for air strikes.

In early January, Fereidoon Abbasi, the head of Iran's Atomic Energy Organisation, said Fordow was safe from any kinds of threat by the enemies.

Tehran says it began the project in 2007, but the UN nuclear watchdog, International Atomic Energy Agency (IAEA), believes design work started in 2006.

The existence of the facility only came to light after it was identified by Western intelligence agencies in September 2009.