The visiting U.S. Secretary of State Condoleezza Rice said on Wednesday the North Atlantic Treaty Organization (NATO) was facing a "real test" in Afghanistan.
"I do think the alliance is facing a real test here," Rice said at a joint news conference after talks with British Foreign Secretary David Miliband.
"But we shouldn't underestimate the transformation that NATO itself has gone through in really learning how to fight this fight," Rice said.
She said the fight against the Taliban and extremists would be long and difficult.
"Our populations need to understand this is not a peacekeeping mission. This is a counter-insurgency problem and that's different," said Rice.
"I think the Taliban has changed tactics and we have to be strong enough to deal with that situation," she added.
"Yes it is hard, and yes it is going to take more time but it's an effort that's very much well worth it," said Rice.
Miliband told reporters that his meeting with Rice was "really excellent and a deep dialogue", saying NATO was "not in Afghanistan to create a colony" but was there to help "an independent government run its own affairs."
Rice's visit to London came as the United States intensified pressure on other European allies to send more troops to Afghanistan in an attempt to prevent the country from slipping into civil war.
Last month, Canadian Prime Minister Stephen Harper said he would pull out of his Canadian troops from Afghan if other members of NATO do not provide more help in the southern region of Kandahar.
The German government last week rejected the U.S. request to provide more troops in southern Afghanistan.
Rice also had talks with British Prime Minister Gordon Brown in Downing Street on a range of issues including the ongoing conflict in Afghanistan.
According to the Downing Street statement, the discussions centered on the role of NATO in the fight against the Taliban and the need to set out a clear strategy at the forthcoming NATO summit in April.
On Wednesday, Brown told MPs at the House of Commons that his priority at the two-day NATO summit would be to see a "burden sharing" agreement among allies operating in Afghanistan that was "fair."
"We have 15 percent of the troops in Afghanistan. Other countries, including Spain and France, have made announcements to add to their troops in Afghanistan. But we need a proper burden sharing -- not only in terms of personnel, but also in terms of helicopters and other equipment," said Brown.
The NATO summit will take place in Bucharest from April 2-4. One item high on the agenda will be the appointment of a high-level coordinator of civilian and military activity after former Liberal Democrat leader Paddy Ashdown declined to take up the post.
Britain currently has over 6,000 soldiers based in Afghanistan, and will increase to around 7,700 over the course of the year. And there are about 87 British soldier killed in Afghanistan since November 2001.
British forces are deployed in Afghanistan in support of the UN authorized, NATO-led International Security Assistance Force mission and as part of the U.S.-led Operation Enduring Freedom.