US President Barack Obama will announce in his State of the Union address Tuesday that he will pull 34,000 US troops home from Afghanistan in the next year, a source familiar with his speech said.

The long-awaited move will effectively halve the size of the current 66,000-strong US force in Afghanistan, ahead of a final withdrawal of most foreign combat troops by the end of 2014.

The source, speaking on condition of anonymity so as not to preempt Obama's remarks, said the president would undertake to complete the withdrawals by around the time of his next State of the Union speech next year.

There were no immediate details of how quickly the drawdown would take place. But a senior Pentagon official told AFP it would be tied to the fighting season in Afghanistan, which runs into the fall.

"The commanders will have discretion on pace and focus will be (on) keeping as many forces in play until after the fighting season," the official told AFP.

Outgoing US Defense Secretary Leon Panetta "supports the plan to continue drawing down our forces over the coming year in Afghanistan," the Pentagon official said.

"The approach you'll hear from the president tonight reflects the best military advice from commanders in the Pentagon and in the field."

A senior US official said that Obama had telephoned Afghan President Hamid Karzai, British Prime Minister David Cameron and German Chancellor Angela Merkel to inform them of his decision.

Debate is also taking place within the administration on the size of a residual force, to train Afghan soldiers and to conduct anti-terror missions, that will remain behind after the formal withdrawal.

Last month, US officials suggested it was theoretically possible that Washington would leave no troops in the country, though some observers saw that move as a negotiating tactic with Karzai in town.

The senior official said that Washington remained committed to a long-term strategic partnership with Afghanistan, and reiterated that talks on a bilateral security agreement were still taking place.

Afghanistan has committed to taking full responsibility for its own security by the end of 2014 and the White House said there are now 352,000 troops in new Afghan security forces, thanks to a broad NATO training effort.

NATO says it will no longer lead combat operations in the next two years, but will provide support to Afghan soldiers.

Obama has made ending the wars in Iraq and Afghanistan the centerpiece of his presidency, and often declares a long decade or more of American war is almost over.

He announced after talks at the White House last month that progress in Afghanistan wrought by the sacrifices of US and allied troops would allow a speeded up transition to take place.

"We've pushed the Taliban out of their strongholds," Obama said.

"And our core objective -- the reason we went to war in the first place -- is now within reach: ensuring that Al-Qaeda can never again use Afghanistan to launch attacks against America."