WASHINGTON—Pentagon war planners have concluded that their largest conventional bomb isn't yet capable of destroying Iran's most heavily fortified underground facilities, and are stepping up efforts to make it more powerful, according to U.S. officials briefed on the plan.

The 30,000-pound "bunker-buster" bomb, known as the Massive Ordnance Penetrator, was specifically designed to take out the hardened fortifications built by Iran and North Korea to cloak their nuclear programs.

But initial tests indicated that the bomb, as currently configured, wouldn't be capable of destroying some of Iran's facilities, either because of their depth or because Tehran has 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.

The push to boost the power of the MOP is part of stepped-up contingency planning for a possible strike against Iran's nuclear program, say U.S. officials.