WASHINGTON – Sen. John Kerry, President Barack Obama's nominee for secretary of state, said Thursday that the United States will "do what we must" to prevent Iran from developing a nuclear weapon even as he signaled that diplomacy remains a viable option with Tehran.

Testifying at his confirmation hearing, and with Senate approval a foregone conclusion, Kerry addressed a range of concerns raised by members of the Foreign Relations Committee, from his past outreach to Syrian President Bashar Assad to GOP concerns about the nomination of Republican former Sen. Chuck Hagel to be defense secretary.

"The president has made it definitive — we will do what we must to prevent Iran from obtaining a nuclear weapon," Kerry said in his opening statement. "I repeat here today: Our policy is not containment. It is prevention, and the clock is ticking on our efforts to secure responsible compliance."