WASHINGTON (AFP) - John Kerry was publicly sworn in as secretary of state Wednesday, vowing to work for peace but pledging to do what is needed to stand up to "extremism, terrorism, chaos and evil."

"I am proud to take on this job because I want to work for peace and because the values and ideals of our nation are really what represents the best of the possibilities of life here on earth," Mr Kerry told the audience.

But he warned that "while my preference is for peaceful resolution to conflict, my journey has also taught me that when remedies are exhausted, we must be prepared to defend our cause and do what is necessary to stand up to extremism, terrorism, chaos and evil."

Mr Kerry was first sworn in as secretary of state at a small, private ceremony on Capitol Hill on Friday, less than two hours after Hillary Clinton stepped down from the job.

On Wednesday, Vice-President Joe Biden administered the oath of office to Kerry, his long-time friend from the days when they were both in the US Senate, at a ceremony attended by former secretary of state Madeleine Albright.

Other senators, including John McCain, were among the audience, listening as Mr Biden praised Mr Kerry's integrity and his credentials to be America's top diplomat.

Mr Biden said he regretted only that Mr Kerry had not been sworn in as president in 2004, after he lost to George W. Bush in the elections.

"How different the world might be today had that occurred?" Mr Biden mused, adding 10 or 12 years ago, he might have been the one being sworn in as secretary of state.

But shaking himself from his reverie, he exclaimed: "It's John Kerry's time again." Mr Kerry's speech was short on any specific foreign policy priorities, although he dismissed critics who maintain America should turn inward as it deals with its own domestic and economic problems.

"This is not a time for America to retreat. This is a time for us to continue to lead," Mr Kerry said.

The world was facing "unparalleled technology, unprecedented growth in the number of young people," as well as "unleashed sectarian strife and religious extremism," he warned.

"Unless we stay vigilant, these forces threaten to unravel whole nation states and create greater pockets of instability than we have seen in recent times. This is our challenge." He urged the United States "to join with other nations, to pool our resources, our talents, our thinking, and to create order where there is none and to fix, or try to fix, what is broken."