LONDON - The Pentagon is taking a close look at its war strategy in Afghanistan in the face of rising violence from an increasingly complex insurgency, Defence Secretary Robert Gates said on Thursday.

In Washington, a senior U.S. official told Reuters the State Department was conducting its own formal review expected to last several weeks, indicating a broader rethink by the Bush administration of its strategy in Afghanistan.

Gates declined to say if a formal or informal review had been launched by the Pentagon but noted the United States had previously examined its strategy in Iraq -- a review that resulted in major changes, including a boost in troop numbers.

"We are taking a close look at it and I don't know whether the results of that will be a significant change in strategy or just some adjustments," Gates told reporters in London.

"You have an overall approach, an overall strategy, but you adjust it continually based on the circumstances that you find," he said.

"We did that in Iraq and we made a change in strategy in Iraq, and we are going to continue to look at the situation in Afghanistan," said Gates, who visited Afghanistan this week before flying to London for a meeting with NATO counterparts.

The State Department review was being led by the department's policy planning staff and the South and Central Asian Affairs bureau that handles Afghanistan, the senior U.S. official said.

The State Department conducted a similar review for Iraq, resulting in more diplomats being sent to Iraq at the same time as the "surge" in U.S. troops began last year.

"We will take a look at what, if any, additional assets we will need as well as how the existing assets are being used," said the official of the review that began a few weeks ago.

"I don't expect it to take long. It will not be a months' long review," he added.

Violence in Afghanistan has risen dramatically over the past two years as Taliban militants and other insurgents have mounted more attacks on NATO and Afghan forces and civilians.

Adm. Mike Mullen, chairman of the U.S. Joint Chiefs of Staff, said last week he was not convinced the United States was winning in Afghanistan.

Gates declined to endorse that assessment.

"I think it's actually more complicated than that," he said.

DIFFERENT CHALLENGES

Gates said northern and western Afghanistan were relatively stable but NATO faced challenges in the volatile south -- the Taliban heartland -- and in the east, where a loose syndicate of insurgent and criminal groups has emerged.

"It's not a centrally controlled Taliban insurgency against the government," he said. "It's a number of different challenges against the government."

Gates also singled out safe havens for insurgents in tribal areas across in the border in Pakistan as a cause for concern.

The United States has stepped up operations there, with six missile attacks by unmanned aircraft and a raid by troops this month.

Mullen said last week he had commissioned a new military strategy for Afghanistan and Pakistan. His review may ultimately carry more weight than one carried out by the administration of President George W. Bush, who leaves office in January.

In Afghanistan, the United States has about 33,000 troops, 13,000 of them in the 47,000 strong NATO-led International Security Assistance Force and the rest under direct U.S. command.

Bush said last week he would send an extra Army brigade -- about 4,000 troops -- to Afghanistan in January. But the U.S. commander of the NATO force said this week he still needed three more brigades plus support units.

A senior U.S. defence official said the Pentagon was looking at a broad range of elements in its Afghan strategy, including how the extra brigade would affect the mission.

One question was whether U.S. forces should take on more combat missions, leaving NATO nations with more restrictions on use of their forces to concentrate on other tasks, said the official, briefing reporters on condition of anonymity.