WASHINGTON - An Israeli official says Prime Minister Benjamin Netanyahu will continue fighting against the nuclear agreement signed by Iran, the United States and five other world powers.

The Israeli leader has been personally lobbying U.S. lawmakers to block the nuclear pact -- but supporters now have the 34 votes in the Senate that would be needed to back President Barack Obama when he vetoes a resolution of disapproval.

The agreement limits Iran's nuclear program in exchange for hundreds of billions of dollars in relief from international sanctions.

In a letter delivered to Congress today, Secretary of State John Kerry offered reassurances about Israel's security -- pointing to the billions of dollars the U.S. has provided to Israel for missile defense and other security assistance.

The letter was sent as Kerry defended the Iran deal in a speech in Philadelphia. He told lawmakers and civil leaders at the National Constitution Center that a defeat of the deal would be a "blow to our nation's credibility and leadership." He pledged that U.S. allies, including Israel, would have the "political and military support they need" to protect themselves from the threat posed by Iran.

Kerry's speech was broadcast live in Iran, an unusual occurrence.