John Kerry on Wednesday vowed to resolve conflicts by peaceful means in the first place as he was publicly sworn in as the US secretary of state.

"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," Kerry said as Vice President Joe Biden administered his oath of office at the State Department.

"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," Kerry added.

The 69-year-old Vietnam veteran succeeded Hillary Clinton as the top American envoy Friday when he was first sworn in at a private ceremony held in the Capitol Hill.

A five-term senator, Kerry was elected to chair the Senate Foreign Relations Committee in December 2008 after then chairman Biden was elected vice president.

As the Democratic nominee, Kerry launched a bid for the White House in 2004 but lost to then incumbent George W. Bush by a margin of 34 electoral votes.

Referring to the episode, Biden said he only regretted that Kerry had not been sworn in as president in 2004.

"How different the world might be today had that occurred?" said the vice president.