BAGRAM, Afghanistan - Defense Secretary Robert Gates said on Wednesday he would consider sending more troops to Afghanistan where US commanders say they expect the Taliban to step up attacks from Pakistani sanctuaries.

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Gates, in Afghanistan to ensure commanders have the resources to counter an expected Taliban offensive in the spring, said it was very important the United States and its allies did not let the success achieved in Afghanistan slip away.

Violence in Afghanistan intensified last year to its bloodiest since US-led forces overthrew the Taliban in 2001.

US military commanders said attacks from Pakistan into Afghanistan had surged, several-fold in some areas, and the violence was expected to increase in the spring and summer.

Gates said he had discussed the situation with the commander of Afghanistan's NATO force, General David Richards, and others.

Asked if the commanders had made a case for more troops, he said: "Yes."

"They've indicated what they can do with different force levels," Gates told reporters at the main US base in Afghanistan, at Bagram, north of Kabul, adding he would take the those ideas back to the US joint chiefs of staff for study.

"At that point I'll make a recommendation to the president."

Asked how many more troops might be sent, he said: "It depends on different scenarios and those are the kinds of decisions that we're going to have to look at."

There are more than 40,000 foreign troops in Afghanistan, the highest level since 2001, about 22,000 of them American.

Gates arrived in Afghanistan late on Monday on his first trip to the country since taking over as defense secretary.

PAKISTANI SANCTUARIES

The surge of violence last year and fears of more when the weather gets warmer this year have thrown the spotlight on infiltration from Pakistan and Pakistan's efforts to stop it.

Gates said on Tuesday cross-border attacks from Pakistan were increasing.

US military officials in Kabul told reporters command and control of the Afghan insurgency came from the Pakistani side of the border, where Pakistani forces have also been battling militants.

Training, financing, recruitment, indoctrination, regeneration and other support activities were also taking place in Pakistan, a US military intelligence official said.

US intelligence chief John Negroponte said last week it would be necessary to eliminate the Taliban safe havens in Pakistan's tribal areas to end the Afghan insurgency.

Pakistan was the main backer of the Taliban during the 1990s but officially stopped helping the hardline Islamists after the September 11 attacks, when it joined the US-led war on terrorism.

But while Pakistan has arrested or killed hundreds of al Qaeda members, including several major figures, Afghanistan and some of its allies say it has failed to take effective action against Taliban leaders, their networks and sanctuaries.

Despite such doubts about Pakistani efforts, a NATO spokesman said on Wednesday efforts were being made to coordinate operations with Pakistan and the killing of a top Taliban commander in a US air strike last month was an example of that.

The Taliban commander killed in the December 19 air strike in the southern Afghan province of Helmand, Akhtar Mohammad Osmani, was the most senior Taliban leader killed by US forces since 2001. "The Pakistanis shared intelligence with us that led to the successful attack on Osmani," NATO spokesman Brigadier Richard Nugee told a news conference in Kabul.

He did not elaborate but also cited a Pakistani attack on a militant camp in its South Waziristan tribal region on Tuesday as an example of efforts to coordinate with Pakistan.

Gates said Pakistan was "an extraordinarily strong ally" of the United States in the war on terrorism but militancy on the Pakistani side of the border would have to be dealt with.