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Mr De Hoop Scheffer has taken his post at a testing time for Nato.

The former Dutch foreign minister has staked the credibility of the alliance on its peacekeeping mission in Afghanistan.

He says the operation is Nato's top priority, and has urged member states to offer more troops and equipment to the mission.

The new secretary general must also decide how to respond to Colin Powell's call for Nato to "do more to support peace and stability in Iraq".

The call comes at a time when relations with the US are fragile due to disagreements over the war in Iraq and plans for a European defence force.

Later this year Mr De Hoop Scheffer will oversee Nato's expansion from 19 to 26 members, as seven former communist bloc countries join the alliance.

Russia's defence minister, Sergei Ivanov, has expressed concern at the move, and says his country will not tolerate the "encroachment" of Nato military infrastructure on its borders.

Should Nato take a greater role in Iraq? Is it right to expand? Is the alliance still relevant in the new world order?