The Pentagon has upgraded and tested the largest bunker-buster bomb in the U.S. arsenal, senior U.S. officials said, readying a weapon that could destroy or disable Iran’s most heavily fortified nuclear facilities should a nuclear deal fall apart and the White House decide to take military action.

Even while the Obama administration was pursuing a diplomatic agreement with Iran to rein in its nuclear program, the Pentagon was readying the improvements to one of its most destructive conventional weapons, including electronic countermeasures to prevent an adversary from jamming its guidance systems.

“The Pentagon continues to be focused on being able to provide military options for Iran if needed,” a senior U.S. official said. “We have not taken our eyes off the ball.” […]