Former Dutch foreign minister Jaap de Hoop Scheffer has taken over as Nato secretary general, replacing Britain's Lord Robertson.
Mr De Hoop Scheffer stood down as Dutch foreign minister to take the job
Mr De Hoop Scheffer said his main priority would be the success of the alliance's mission in Afghanistan.
He also pledged to help "build bridges" between the US and Europe following strained ties over the Iraq war.
Mr De Hoop Scheffer, 55, has returned to Nato 20 years after working there as a Dutch diplomat.
He takes up the post as the head of the world's biggest military alliance at a turning point in its history. Later this year, the 19-member bloc will admit seven new members from eastern Europe.
"The primary focus at the moment should be on Afghanistan," Mr De Hoop Scheffer said as he arrived at Nato headquarters in Brussels on Monday.
"Iraq will of course be on the agenda at a certain stage, but let's take the events step by step."
Last year, Nato went out of its traditional European area of operations for the first time to carry out a peacekeeping mission in the Afghan capital Kabul. The mission has since been extended to the northern city of Kunduz and could be further boosted this year.
The BBC's Oana Lungescu in Brussels says the Afghan mission is a serious test of Nato's credibility.
Trans-Atlantic relations within the alliance were badly damaged by the divisions over Iraq, with France and Germany strongly opposed to the US-led invasion.
Tensions have also emerged over US concerns at European Union plans to create its own military command.
"Relations between the European Union and the United States will be one of my prime targets," Mr De Hoop Scheffer said.
Correspondents say key decisions for the future of the alliance will be taken at a summit in Istanbul in late June, when Nato is expected to declare its mission in Bosnia a success, setting the scene for a handover to EU peace-keepers.
Mr De Hoop Scheffer will also need to press ahead with Lord Robertson's campaign to modernise Europe's armed forces.