Photo: The Canadian Press. All rights reserved.

FILE - In this Sept. 18, 2012 file photo, female soldiers from 1st Brigade Combat Team, 101st Airborne Division train on a firing range while testing new body armor in Fort Campbell, Ky., in preparation for their deployment to Afghanistan. The Pentagon is lifting its ban on women serving in combat, opening hundreds of thousands of front-line positions and potentially elite commando jobs after generations of limits on their service, defense officials said Wednesday, Jan. 23, 2013. (AP Photo/Mark Humphrey, File)

The Pentagon is lifting its ban on women serving in combat, opening hundreds of thousands of front-line positions and potentially elite commando jobs after generations of limits on their service, defence officials said Wednesday.

The changes, set to be announced Thursday by Defence Secretary Leon Panetta, will not happen overnight. The services must now develop plans for allowing women to seek the combat positions, a senior military official said. Some jobs may open as soon as this year, while assessments for others, such as special operations forces, including Navy SEALS and the Army's Delta Force, may take longer. The services will have until January 2016 to make a case to that some positions should remain closed to women.

The groundbreaking move recommended by the Joint Chiefs of Staff overturns a 1994 rule prohibiting women from being assigned to smaller ground combat units.

Officials briefed The Associated Press on the changes on condition of anonymity so they could speak ahead of the official announcement.

There long has been opposition to putting women in combat, based on questions of whether they have the necessary strength and stamina for certain jobs, or whether their presence might hurt unit cohesion.

But as news of Panetta's expected order got out, members of Congress, including the Senate Armed Services Committee chairman, Democratic Sen. Carl Levin, announced their support.

"It reflects the reality of 21st century military operations," Levin said.