South Korea's Ban Ki-moon has been sworn in as the next UN secretary general at a ceremony in New York.
Mr Ban, from South Korea, will take up his post as the eighth secretary general on 1 January, 2007.
He told ambassadors he would be a "bridge-builder", leading by example as he sought to restore trust in a UN that needed to be "dynamic and courageous".
He added his voice to tributes paid to outgoing Secretary General Kofi Annan, saying he was humbled to follow him.
Mr Ban's oath of office was administered by General Assembly President Haya Rashed al-Khalifa, from Bahrain.
She said she was confident he would "lead the organisation wisely, with determination and integrity".
In a statement made to the General Assembly after taking the oath, Mr Ban pledged to work to rebuild the trust of member states in the organisation.
"By strengthening the three pillars of our United Nations - security, development and human rights - we can build a more peaceful, more prosperous and more just world for succeeding generations," he said.
"As we pursue our collective endeavour to reach that goal, my first priority will be to restore trust. I will seek to act as a harmoniser and bridge-builder."
He said he intended to set the highest ethical standards to protect the UN's reputation.
"The good name of the United Nations is one of its most valuable assets - but also one of its most vulnerable," he said.
The BBC's Laura Trevelyan in New York says Mr Ban is keen to improve morale at the UN, which has been hit hard by the oil-for-food scandal.
An investigation last year found that the oil-for-food scheme operated by the UN with Saddam Hussein's Iraq had been mismanaged and was riddled with corruption.
North Korea talks
Giving a press conference after the ceremony, Mr Ban said the conflict in the Middle East was one of the most serious issues facing the world.
Mr Ban will be the first Asian UN secretary general for 35 years
He also said he would focus on the situation in Sudan, saying the suffering of the people of Darfur was "simply unacceptable".
However, he said there was no military solution and that the UN, African Union and Sudan must continue to talk.
Mr Ban said he would watch closely the resumption of six-party talks on North Korea's nuclear programme next week.
He added that it was "crucially important" that North Korea live up to commitment made last September to give up all its nuclear activities.
Unanimously elected in October, Mr Ban will be the first Asian secretary general for 35 years.
He has said he is considering several potential contenders for the role of deputy secretary general, with a preference given to women candidates.
Mr Annan, who received a standing ovation from 192-member assembly, was praised for his efforts during his 10 years in office.
A resolution hailed "his exceptional contribution to international peace and security, as well as his outstanding efforts to strengthen the United Nations system and promote and protect human rights and fundamental freedoms for all, in the interest of a better world".
The assembly paid tribute to Kofi Annan's achievements as UN head
Ms Khalifa also paid tribute to the 68-year-old's achievements.
"Kofi Annan will leave a lasting legacy. He has guided the United Nations into the 21st Century with vision and leadership. As a result, the multilateral system is stronger," she said.
On Tuesday, Mr Annan used his final speech as UN secretary general to call on the US not to lose sight of its core principles in its fight on terror.
Born in Ghana in 1938, Mr Annan has led the UN since 1997 and, in 2001, he and the UN received the Nobel Peace Prize.