"NATO remains a cornerstone of America's relations with Europe, and a vital player in our efforts to deal with the security challenges of the 21st century," Scott McClellan said in a statement.

"Under Lord Robertson's leadership, NATO provided support to Turkey during the Iraq war, and has now agreed to take over the leadership of the International Security Assistance Force in Afghanistan later this summer."

Robertson was to visit Washington on Sunday and Monday for meetings with Bush, Secretary of Defense Donald Rumsfeld and Secretary of State Colin Powell, a NATO statement said.

On Tuesday the British secretary-general will head on to Montreal and Ottawa, where he will have talks with Canadian Prime Minister Jean Chretien and Minister of Defense John McCallum.

The trip, which also includes a Washington conference on NATO's relations with Ukraine, comes after the defence summit Tuesday by France, Germany, Luxembourg and Belgium -- which all opposed the US-British war in Iraq.

The quartet called for deeper military integration by proposing their own command headquarters for military operations outside the NATO arena.

The United States and Britain reacted frostily to the initiative. British Foreign Secretary Jack Straw said Friday that plans to boost Europe's defence capabilities must underpin NATO and not sidetrack the 19-nation alliance.

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