President Barack Obama will order his promised US troop drawdown from Afghanistan in a prime time address Wednesday, which one official said would likely see 10,000 soldiers called home this year.

US soldiers stand guard on a watch tower overlooking villages at Combat Outpost (COP) Sabari in Khost province in the east of Afghanistan on June 21, 2011.

Obama will stick with his vow to begin pulling out US forces after an 18-month troop surge, but apparently heed Pentagon warnings that an overly swift withdrawal could imperil hard won gains against Taliban insurgents.

His broadcast at 8:00 pm (0000 GMT) will take place as political attitudes shift on the war following the killing of Osama bin Laden, other heavy US blows against Al-Qaeda and as US public support ebbs for the grueling conflict.

"The president will address the nation from the White House to lay out his plan for implementing his strategy -- first unveiled in December 2009 -- to draw down American troops from Afghanistan," Obama spokesman Jay Carney said.

Hours before his address, six Afghan police were killed Wednesday when Taliban attacked their checkpoint, a brazen assault likely to raise fresh security questions.

The Taliban claimed responsibility for the attack in Qarabagh district, southwest of Kabul.

"The attack started at around 7:00 am (2:30 GMT) and six police officers were killed during a gunfight with the attackers," Sayed Amir Shah, the head of Afghanistan\'s intelligence agency in Ghazni province, told AFP. "The gunfire is still ongoing," Shah said.

A senior defense official said on condition of anonymity that the president would "likely" order the withdrawal of about 10,000 troops from Afghanistan this year. Half would come out this summer and half by the end of 2011.

Another 20,000 troops, part of a 30,000 strong surge ordered in December 2009, would be withdrawn by the end of next year, meaning elevated force levels would remain through two more Afghan summer fighting seasons.

There are currently 99,000 US soldiers in Afghanistan.

The official, speaking to AFP on condition of anonymity, described the numbers as the most "likely" option to be backed by Obama, who has nearly tripled US force strength in Afghanistan since taking office.

Carney branded a flurry of media reports on the size of the drawdown as mere speculation and refused to divulge details of the president\'s decision, which was finalized earlier on Tuesday.

Obama promised Americans when he unveiled the surge that he would begin to bring soldiers home in July 2011 -- a vow critics panned as offering succor to US enemies.

The months leading up to the drawdown decision have seen a turf war that has drawn in Obama\'s political team, Pentagon brass and lawmakers keenly aware of voter fatigue over a decade of war in Iraq and Afghanistan.

Democratic Senator Carl Levin reiterated his argument that improved security conditions would permit Obama to bring home 15,000 combat troops by the end of the year.

"The level of US troop reductions in Afghanistan needs to be significant to achieve its purpose --- letting the Afghan government know we are determined to shift primary responsibility for their security to the Afghan security forces," Levin said.

Another powerful Democrat, Senator John Kerry, said Obama needed to change the US emphasis in Afghanistan.

"We\'ve been enormously successful in stripping Al-Qaeda of its capacity. And we need to reflect that now in the troop presence and in the definition of the mission."

Obama\'s speech will come less than two months after a daring US forces raid into Pakistan killed Al-Qaeda chief bin Laden, the prime target of the US war on terror launched after the September 11 attacks in 2001.

Since the Afghan mission was partly defined as an effort to disrupt, dismantle and defeat Al-Qaeda, some war critics have questioned why America needs to keep such large troop numbers in Afghanistan.

Carney, however, argued that a key component of the mission was the need to stabilize Afghanistan so it would not revert to a haven for terror groups and could meet a 2014 deadline to assume control of its own security.

Obama is expected to explain that his surge has wrought slow, but significant progress, particularly in southern Afghanistan against the Taliban.

But US discontent on the war is steadily rising, and Obama will be keenly aware of public opinion as he plots his 2012 reelection race.

In a new Pew Research Center survey, 56 percent of respondents said US troops should be brought home as soon as possible, while 39 percent said they should stay in Afghanistan until the situation had stabilized.

Skeptics argue that after the deaths of more than 1,600 US troops and at a cost of nearly $10 billion a month, the conflict is unsustainable.

The decision on withdrawal was complicated by the fragile outreach process that Washington has launched with the Taliban.

Backers of the surge have said the Taliban will only countenance serious reconciliation talks if the group\'s commanders feel they are under pressure from a strong US military footprint in Afghanistan.