WASHINGTON — 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.

The changes, set to be announced Thursday by Defense 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.

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, Sen. Carl Levin, D-Mich., announced their support.

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

Sen. Jim Inhofe of Oklahoma, who will be the top Republican on the Armed Services panel, said, however, that he does not believe this will be a broad opening of combat roles for women because there are practical barriers that have to be overcome in order to protect the safety and privacy of all members of the military.

Panetta’s move comes in his final weeks as Pentagon chief and just days after President Barack Obama’s inaugural speech in which he spoke passionately about equal rights for all. Panetta’s decision could open more than 230,000 jobs, many in Army and Marine infantry units, to women.