U.S. Vice-President Dick Cheney made an unannounced visit to Afghanistan on Thursday to meet President Hamid Karzai as the United States urges NATO allies to provide more troops and support.

President George W. Bush asked Cheney, who is on a Middle East trip that began with a stop in Iraq, to meet Karzai in advance of a NATO summit "to discuss progress in a democratic Afghanistan, as well as the work that lies ahead, especially in the south", Lea Anne McBride, spokeswoman for Cheney, told reporters travelling with him.

The Afghan mission is considered by many analysts as the toughest ground war faced by the 59-year-old alliance and has led to open differences among allies over tactics and troop levels, and it will be a key issue at the NATO summit in Bucharest in early April.

NATO's International Security Assistance Force in Afghanistan has about 43,000 troops.

"The vice president will discuss the existing U.S.-Afghanistan strategic partnership and how we will continue our efforts to fight terrorism and will help Afghanistan further along the road toward becoming a more prosperous and stable country," McBride said.

Taliban Islamist militants have threatened to step up suicide attacks on the capital Kabul this year in a campaign to wear down the will of NATO countries to carry on the fight in Afghanistan and force a withdrawal of foreign troops.

NATO is struggling to come up with more troops, with some European members reluctant to send their forces to southern and eastern Afghanistan where U.S., British, Canadian and Dutch soldiers clash almost daily with Taliban militants.

Canada, with 2,500 troops in southern Afghanistan, wants NATO allies to provide another 1,000 soldiers to reinforce its combat forces as a condition for keeping its troops in the country.

Cheney will meet U.S. troops in Afghanistan.