President Barack Obama congratulated the Afghan people on Friday for pulling off a presidential election in the midst of violent intimidation by Taliban militants but cautioned that more difficult days lie ahead for the war-weary nation.

``This is an important step forward in the Afghan people's efforts to take control of their future even as violent extremists are trying to stand in their way,'' Mr Obama said at the White House before boarding Marine One for a flight to the presidential mountaintop retreat at Camp David, Md.

In Kabul, the government's chief electoral officer, Daoud Ali Najafi, said results from Thursday's balloting won't be made public until Tuesday.

Mr Obama said it was the first democratic election run by Afghans in more than three decades. The 2004 election that put President Hamid Karzai in power was run by the United Nations. Karzai and top challenger Abdullah Abdullah each described himself Friday as the likely winner of Thursday's voting, although many believed they would be forced to compete in a runoff election in October.

The United States has been steadily increasing its military efforts in Afghanistan since Obama took office, arguing that that is a far more effective use of American troops than the Bush administration's emphasis on the war in Iraq. U.S. and allied casualties also have mounted, particularly since 2007.

Mr Obama said it was obvious in advance of the election that the Taliban, who control substantial portions of the country, particularly in the volatile south, would attempt to disrupt the voting.

``Over the last few days, in particular yesterday, we've seen acts of violence and intimidation by the Taliban, and there may be more in the days to come,'' he said.

Regardless of the ultimate winner of the presidential election, the U.S. will continue to work with Afghans to stabilize the country, Mr Obama said.

``Our goal is clear: to disrupt, dismantle and defeat al-Qaeda and their extremist allies,'' he said. ``That goal will be achieved, and our troops will be able to come home as Afghans continue to strengthen their own capacity and take responsibility for their own future.''

With memories of the Sept. 11, 2001, terrorist attacks dimming, Americans are growing weary of the conflict. New polling this week showed a majority _ 51 percent _ of those surveyed now believe the war is not worth the fight, an increase of 6 percentage points in a month.

Still, a White House strategy review is due in mid-September, and Gen. Stanley McChrystal, the top U.S. commander in Afghanistan, is expected to press for a further increase in forces for his counterinsurgency campaign.

Sen. Bob Corker, a Republican who was in Afghanistan to observe Thursday's elections, told CNN television's Wolf Blitzer on Friday that more U.S. troops probably would be needed, given the importance of security in moving Afghanistan toward political stability.

``The fact that security was more of a concern during this election than the 2004 election obviously says a lot about what's occurred here over the last five years,'' Mr Corker said in an interview from Pakistan. He was referring to the deterioration of security as the Taliban has gained strength in some areas.

Just three years ago the U.S. had about 20,000 forces in the country. Today, it has triple that, on the way to 68,000 by year's end when all of the extra 17,000 troops that Mr Obama announced in March are to be in place. An additional 4,000 troops are arriving to help train Afghan forces.

Mr Obama has not wavered from his campaign pledge to take the fight

to the Taliban and their al-Qaeda allies in Afghanistan and Pakistan. He argues that the true danger to Americans lies in the towering peaks and vast deserts of those countries, not in Iraq. The Bush administration, he asserts, wasted precious time, treasure and blood in toppling Saddam Hussein and getting caught up in an insurgency.

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