President Obama will announce at Tuesday night's State of the Union address that 34,000 troops - more than half of those currently serving in the combat region - will be back from Afghanistan a year from tonight, according to a source familiar with the speech.

Roughly 66,000 U.S. troops are serving in Afghanistan. The military has proposed keeping several thousand troops in the country after 2014 as advisers, trainers and logistical support for Afghan forces; the White House has said it remains open to pulling out all troops entirely.

Tune in to ABCNews.com/live for live streaming coverage of the 2013 State of the Union Address at 9 p.m. ET

In January, during a meeting with Afghan President Hamid Karzai, Obama said today that most U.S. combat operations in Afghanistan would end this spring, signaling a quickening troop drawdown that will bring the decade-long war to a close at the end of 2014.

"Our troops will continue to fight alongside Afghans when needed, but let me say it as plainly as I can: Starting this spring, our troops will have a different mission - training, advising, assisting Afghan forces," Obama said Jan. 11 th at an East Room news conference in Washington.

His comments planned for the State of the Union Address put specific numbers along with that new mission.

Update at 3:06 p.m. ET

Pentagon officials have repeatedly said that before they could calculate the pace with which 66,000 U.S. troops would leave Afghanistan they had to determine how many would be left behind after 2014. But tonight's planned announcement from President Obama would seem to reverse that rationale.

A U.S. official explains that it was determined that announcing a troop reduction wouldn't impact the analysis of how many troops are left after 2014. The official said it was made easier by the fact that the "mission-sets " for that future force are already pretty clear and there didn't seem to be a conflict. The official says the 34,000 number was recommended to the President by both outgoing Defense Secretary Leon Panetta and Gen. John Allen in Afghanistan.

Another U.S. official says the release of the number today in advance of tonight's speech was done out of concern that the information was about to be reported based on leaks.

A U.S. official confirms a Washington Post report today that the current White House discussion centers on leaving fewer than 10,000 troops behind in Afghanistan after 2014 and then phasing them down gradually the next few years.

Read More: Obama and Karzai Spell Out Afghanistan Transition

Read More: State of the Union 2013