US Defense Secretary Robert Gates declared Afghanistan the top US military priority Tuesday but said US objectives there should be "limited."

"My own personal view is that our primary goal is to prevent Afghanistan from being used as a base for terrorists and extremists to attack the United States and our allies," he said.

"And whatever else we need to do flows from that objective," he told lawmakers in his first appearance before the Senate Armed Services Committee as President Barack Obama's defense secretary.

His comments marked a significant narrowing of US ambitions even as the United States prepares to nearly double the size of its forces in Afghanistan in response to an unraveling security situation.

"There is little doubt that our greatest military challenge right now is Afghanistan," Gates said in his opening statement. "President Obama has made it clear that the Afghanistan theater should be our top military priority."

Gates told lawmakers that the bulk of a 30,000-troop buildup requested by the US commander in Afghanistan, General David McKiernan, could be in place by mid-summer but bases must be expanded to receive the full complement of additional forces.

"But I would be very skeptical of additional forces levels, American force levels, beyond what General McKiernan has already asked for," he said.

Obama meets Wednesday at the Pentagon with the military chiefs to review planning for Afghanistan and a parallel drawdown of the 142,000 US troops in Iraq.

In testimony in the Senate and later at the House of Representatives, Gates said the military will present Obama with a range of options, including the withdrawal of all US combat brigades from Iraq in 16 months.

"He has been firm in saying he wants a responsible withdrawal and one that is safe for our troops," Gates told the House Armed Services Committee.

"I think this is a very thorough and a very real process. I don't think anybody associated with it in the Department of Defense feels like we're going through the motions; that a decision has already been made," he said.

Meanwhile, Admiral Michael Mullen, the chairman of the Joint Chiefs of Staff, called on European allies to provide more troops and funding for Afghanistan.

Observing that the NATO-led force has grown by 10,000 troops over the past year, Mullen said, "We need those contributions and we will need more."

Gates said the US goal in Afghanistan was a state in which the Afghan people do not provide a safe haven to al-Qaeda, reject the rule of the Taliban and support their legitimately elected government.

But he was blunt about the difficulties of stabilizing the country and the chances for success of an extensive nation-building project.

"If we set ourselves the objective of creating some sort of a Central Asian Valhalla over there, we will lose," he warned Senators. "Because nobody in the world has that much time, patience or money, to be honest."

"It seems to me we ought to keep our objectives realistic and limited in Afghanistan. Otherwise we will set ourselves up for failure," he said.

Valhalla is commonly used as a synonym for heaven, but in old Norse mythology it was a great hall where heroes slain in battle are received.

Gates also acknowledged that civilian casualties from US air strikes "are doing enormous harm to us in Afghanistan."

"We have to got to figure out better ways to do these things or have Afghans in the lead, because my fear is that the Afghans come to see us as part of the problem instead of part of the solution," he said.

But Gates defended stepped up air strikes by US unmanned aircraft along the border with Pakistan in tribal areas that Al-Qaeda, the Taliban and other militants have used as a safe haven.

The United States will continue to pursue Al-Qaeda "anywhere it is," Gates said, telling lawmakers that decision has been transmitted to the government of Pakistan.