The violence that has surged for two years in Afghanistan reached a new high last week, and more difficulty lies ahead, the United States' top war zone commander said Thursday.

Gen. David Petraeus said the number of attacks in Afghanistan over the last week hit the highest level since the December 2001 fall of the Taliban.

"Some of this will go up because we are going to go after their sanctuaries and safe havens as we must," Petraeus, head of U.S. Central Command, said during a speech at the Washington think-tank Center for a New American Security.

"But there is no question the situation has deteriorated over the course of the past two years in particular and there are difficult times ahead," he said.

There were more than 400 insurgent attacks last week, including ambushes, small arms volleys, assaults on Afghan infrastructure and government offices, and roadside bomb and mine explosions. In comparison, attacks in January 2004 were less than 50 per week.

Extremist attacks in the rural nation tend to increase in the summer months, and in part are spurred by military efforts to crack down on insurgents, Petraeus said.

Petraeus, who led a beefed-up U.S. military effort that helped turnabout violence in Iraq in 2007, noted several challenges in Afghanistan he did not face while in Baghdad — including the inability of U.S. troops to live among the local residents.

He cited the need for "being good partners and good neighbors and having enormous concern, needless to say, about civilian casualties in everything we do."

Petraeus did not shed light on an inquiry he is overseeing about U.S. airstrikes in a western Afghanistan province that killed at least 30 civilians while trying to protect a village from a Taliban onslaught.

The inquiry's results are to be released as early as Friday, but the Pentagon earlier this week said U.S. troops did not follow proper tactics and procedures during the May 4-5 assault in Farah province.

Afghan officials have said that the civilian toll was 140 dead, but U.S. commanders have said they believe no more than 30 civilians were killed, along with 60 to 65 Taliban insurgents.

Pentagon spokesman Geoff Morrell said Monday that it is unclear whether an apparent failure by a B-1 bomber crew to follow procedures during the strikes "had anything to do with the fact that civilian casualties did occur in this incident."

In Brussels, after meeting Thursday with European defense ministers, Defense Sec. Robert Gates said that NATO allies agree with the United States that progress must be shown in Afghanistan over the next year to 18 months.

Gates said that after this summer's election in Afghanistan, he hopes "to see if we can shift the momentum."

Speaking to reporters traveling with him from the Netherlands to Belgium, Gates said: "If we can show we're making progress, if we're heading in the right direction, the American people and the Congress will sustain this effort. But if in a year or so it appears we are in a stalemate and we're taking even more casualties, that patience would wear thin pretty soon."

Gates said he wants a wide role for the military alliance in a rearranged hierarchy that will include his new team of hand-picked generals. Gen. Stanley McChrystal, Gates' choice as the top Afghanistan commander, is expected in Europe this week.

The Pentagon is giving McChrystal as many as 400 officers and others as a brain trust devoted to Afghanistan and the related problem of instability and porous borders in Pakistan.

The alliance will cut its 14,000-member Kosovo peacekeeping force by a third as security improves in that newly independent nation, NATO Secretary-General Jaap de Hoop Scheffer said Thursday.

Gates said he hopes the scaling back of security in Kosovo might free up additional soldiers for the Afghan fight.

Besides Kosovo, the naval anti-piracy mission off Somalia's coastline also figured prominently on the agenda of the first day of the two-day meeting, de Hoop Scheffer said.