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...