New UN Secretary General Ban Ki-moon has begun work with a pledge to pay the highest attention to the conflict in Sudan's Darfur region.
Mr Ban, 62, is the first Asian to hold the top post in 35 years
Mr Ban said his first trip would be to the African Union summit in Ethiopia this month where he hopes to meet Sudan's President Omar al-Bashir.
Staff lined up to applaud Mr Ban as he entered the UN offices in New York.
The South Korean ex-foreign minister succeeds Kofi Annan, who was at the helm for 10 years.
'In the front line'
Mr Ban entered the building to an honour guard and first went to the meditation room to pay tribute to peace keepers who have died while on duty.
Afterwards he said he would turn his attention immediately to the "crisis situation in Darfur". More than 300,000 people have died there in the past three years.
More than 300,000 have died in Darfur in the past three years
The UN has a three-part plan to strengthen the current 7,000-strong African Union force in Darfur with UN troops.
President Bashir has consistently opposed any large UN deployment although he has indicated he wants to support the UN's plans.
Mr Ban said the large media presence for his appearance at work was "vivid proof that the United Nations is much alive and [in the] front line addressing all the challenges and issues".
He added: "It is a force to be reckoned with among governments and people everywhere."
Mr Ban vowed to give priority too to the issue of North Korea's nuclear programme and to defend human rights.
He also spoke about the execution of former Iraqi leader Saddam Hussein.
Mr Ban said Saddam Hussein was responsible for unspeakable atrocities and that capital punishment was for individual countries to decide upon.
But he added: "I also hope that members of the international community should pay due regard to all aspects of international humanitarian laws."
Mr Ban, 62, is the first Asian to hold the top post in 35 years.