“The 30,000-pound [13,600 kilograms] "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 programs,” the daily 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.

U.S. Defense 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," Mr. Panetta said.

U.S. Officials say new money was meant to 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 antiaircraft batteries, which makes it a very difficult target for air strikes.

Organization (AEOI) Fereidoon Abbasi said Frodow is safe from any kinds of threat by the enemies.

Tehran said it began the project in 2007, but the 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.