The United States will withdraw 33,000 Americans troops from Afghanistan by the summer of 2012, beginning with an initial drawdown of 10,000 surge forces by this year-end, President Barack Obama has said.

"Starting next month, we will be able to remove 10,000 of our troops from Afghanistan by the end of this year, and we will bring home a total of 33,000 troops by next summer, fully recovering the surge I announced at West Point (in December 2009)," Obama said in a nationally televised speech from the White House late on Wednesday night.

"After this initial reduction, our troops will continue coming home at a steady pace as Afghan security forces move into the lead. Our mission will change from combat to support. By 2014, this process of transition will be complete, and the Afghan people will be responsible for their own security," he said.

The US President said America was starting the drawdown from a position of strength, asserting that the Al Qaeda was under "more pressure" than at any time since 9/11.

"Together with the Pakistanis, we have taken out more than half of Al Qaeda's leadership. And thanks to our intelligence professionals and Special Forces, we killed Osama bin Laden, the only leader that Al Qaeda had ever known," he said.

Obama said nearly ten years ago, America suffered the "worst attack" on its shores since Pearl Harbour.

"This mass murder was planned by Osama bin Laden and his Al Qaeda network in Afghanistan, and signaled a new threat to our security-- one in which the targets were no longer soldiers on a battlefield, but innocent men, women and children going about their daily lives," he said.

"In the days that followed, our nation was united as we struck at Al Qaeda and routed the Taliban in Afghanistan. Then, our focus shifted. A second war was launched in Iraq, and we spent enormous blood and treasure to support a new government there. By the time I took office, the war in Afghanistan had entered its seventh year," he said.

"But Al Qaeda's leaders had escaped into Pakistan and were plotting new attacks, while the Taliban had regrouped and gone on the offensive. Without a new strategy and decisive action, our military commanders warned that we could face a resurgent Al Qaeda, and a Taliban taking over large parts of Afghanistan," the President said.

For this reason, Obama said, "In one of the most difficult decisions that I've made as President, I ordered an additional 30,000 American troops into Afghanistan."

He said when he had announced this surge at West Point, US had set clear objectives -- to refocus on Al Qaeda, reverse the Taliban's momentum and train Afghan security forces to defend their own country.

"I also made it clear that our commitment would not be open-ended, and that we would begin to drawdown our forces this July," he said.