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."

Pressed on Iran and its nuclear ambitions, Kerry said he was hopeful that the U.S. and other nations could make progress on the diplomatic front, but that Tehran needs to understand it must prove that its program is for peaceful purposes.

"It is not hard to prove," he said, stressing that "intrusive inspections" are required.

In an unexpected exchange, Kerry found himself defending Obama's controversial pick of Hagel to be the next defense secretary against GOP criticism.

Sen. Bob Corker, the senior Republican on the panel, expressed concerns about Hagel's support for an 80 percent reduction of U.S. nuclear weapons, a major issue for the Tennessee lawmaker and his home state. The Y-12 nuclear facility is located near Oak Ridge, Tenn., and any cuts would have an impact on local jobs.

"I know Chuck Hagel. I think he is a strong patriotic former senator, and he will be a strong secretary of defense," Kerry said of Hagel, who like Kerry served in Vietnam.

The Massachusetts senator urged lawmakers to be realistic, arguing that an 80 percent cut is an aspiration that would be unlikely in the current climate.

On Syria, Kerry was asked about his outreach to Assad, now an international pariah after months of civil war and unending violence against his citizens.

Kerry said there was a moment where Syria reached out to the West but that the moment has long passed.

"History caught up to us. That never happened. And it's now moot, because he (Assad) has made a set of judgments that are inexcusable, that are reprehensible, and I think is not long for remaining as the head of state in Syria," the senator said. "I think the time is ticking."

Sen. John McCain, R-Ariz., a fierce critic of Obama's policy on Syria, said the status quo is unacceptable with the United Nations estimating that 60,000 have been killed and the heavy influx of refugees in Jordan and Turkey.

After a recent visit to the refugee camps, McCain warned that Syrians frustrated with the U.S. response will be a recruitment target for extremists.

"We can do a lot more without putting American boots on the ground," McCain said. "Otherwise, we will be judged harshly by history."

Kerry said it was imperative to continue discussions with Russia and others in dealing with Syria, but he was realistic.

"I don't have optimism," he said.

The hearing was an odd juxtaposition. Kerry has served on the committee during his entire 28 years in the Senate and has chaired the panel for the last four. On Thursday, he sat at the witness table, his voice at times cracking from emotion, facing his colleagues and friends.

Obama chose Kerry to succeed Secretary of State Hillary Rodham Clinton, who introduced the senator. "John is the right choice," Clinton told the panel. "He will bring a record of leadership and service that is exemplary."

Sen. Bob Menendez, D-N.J., the incoming chairman who presided over the confirmation hearing, noted that Kerry was the first senator on the panel in a century to ascend to the Cabinet post. Corker told Kerry, "you've almost lived your entire life for this moment."

A lone protester shouting about the Middle East interrupted the hearing. Just as Kerry completed his prepared testimony, the woman began shouting about the Middle East and was escorted from the room.

The Vietnam War, a long, bitter conflict that decimated a generation of draft-age men, played a prominent role at the hearing.

In his testimony, Kerry alluded to his controversial moment before the committee some 42 years ago, when the decorated Vietnam veteran testified about his opposition to the war, and famously asked, "How do you ask a man to be the last man to die for a mistake?"

"Today I can't help but recognize that the world itself then was in many ways simpler, divided as it was along bi-polar, Cold War antagonisms," Kerry said. "Today's world is more complicated than anything we have experienced - from the emergence of China to the Arab Awakening: inextricably linked economic, health, environmental and demographic issues" as well as issues such as proliferation.

McCain, who also introduced Kerry, said their friendship took root with their work on a committee seeking to resolve the status of POWs and missing in action from Vietnam as well as efforts to ensure normal U.S. relations with Vietnam during President Bill Clinton's administration.

"Helping to establish a relationship with Vietnam that serves American interests and values, rather than one that remained mired in mutual resentment and bitterness, is one of my proudest accomplishments as a senator, and I expect it is one of John's as well," McCain said. "Working toward that end with John, and witnessing almost daily his exemplary statesmanship, is one of the highest privileges I've had here."

The hearing is the first of three for Obama's national security nominees, and the least controversial.

Hagel will face tough questions about his past statements on Israel, Iran, nuclear weapons and defense spending at his confirmation hearing next Thursday before the Senate Armed Services Committee. John Brennan, the president's choice for CIA director, will be quizzed about White House national security leaks and the use of unmanned drones at his hearing next month.

The job of the nation's top diplomat would be the realization of a dream for Kerry, whom Obama passed over in 2008 when he chose Clinton. When Joe Biden became vice president, Kerry replaced the former Delaware senator as chairman of the committee.

Obama nominated Kerry after Susan Rice, the U.S. ambassador to the United Nations, removed her name from consideration following criticism from Republicans over her initial comments about the attacks on the U.S. Consulate in Benghazi, Libya.

Kerry, 69, is the son of a diplomat and has served as Obama's unofficial envoy, using his skills of persuasion with leaders of Afghanistan and Pakistan.

In advance of the hearing, Kerry said he plans to divest holdings in dozens of companies in his family's vast financial portfolio to avoid conflicts of interest if he is confirmed.

He notified the State Department earlier this month that within 90 days of his confirmation he would move to sell off holdings in three trusts benefiting him and his wife, Teresa Heinz Kerry. In the Jan. 8 letter to the department's Office of the Legal Adviser, Kerry said he would not take part in any decisions that could affect the companies he has holdings in until those investments are sold off.

Kerry is the wealthiest man in the Senate, worth more than $184 million, according to a 2011 Senate disclosure.