Secretary of Defense Robert Gates, left, listens to Chairman of the Joint Chiefs of Staff Gen. Peter Pace. The two spoke Thursday at the Pentagon during their weekly news conference on the status of U.S. efforts in Iraq.

Washington ? Defense Secretary Robert Gates said Thursday he does not anticipate extending U.S. troop deployments in Iraq beyond 15 months, calling the idea a “worst-case scenario.”

Gates endorsed efforts to work with some Iraqi insurgents who initially fought against U.S. forces. That may be the only way to bring peace to the bitterly divided nation, he said.

“If we refuse to work with or ally with everybody who’s been on the other side of the fence, then the prospects for making any progress in Iraq are pretty slim,” Gates said at a Pentagon news briefing. “Trying to bring some level of peace to Iraq is trying to persuade some people who have been fighting to stop fighting and become a part of a political process.”

Gates and Gen. Peter Pace, chairman of the Joint Chiefs of Staff, acknowledged there was some risk involved in arming groups that have attacked U.S. troops in the past. But they said the strategy has worked well in the Anbar province.

“I think there’s a greater risk of missed opportunities : that we should seize on those opportunities,” Pace said.

Gates also said he couldn’t say how long American forces would have to stay at increased levels in the effort to secure Baghdad. There are 156,000 U.S. troops in Iraq, as the final brigade in the nearly 30,000-troop buildup has moved into Iraq and begun operations.

Military deployments were recently extended to a maximum of 15 months from 12 months. Gates said it was his hope to move as soon as possible back to 12-month deployments, with a year at home, and then eventually to 12-month terms with two years at home.

But he also said that a recent report’s recommendation that troops get one month off for every three they serve on the front lines would be “a challenge to manage” at this time.

Asked about a recent upswing in attacks on U.S. troops in Iraq, Pace said that as forces move into new sectors to flush out insurgents, there will be tough fighting ahead.

“So it is an expectation that this surge is going to result in more contact and, therefore, more casualties,” Pace said.

He said the current U.S.-led offensive was “exactly what needs to be done,” adding that it “buys time for the Iraqi government. : This is the right thing to do.”

Pace said that while the overall number of attacks against American forces has gone up, that is largely because the number of U.S. brigades has increased from 15 at the start of the year to 20. He said the number of attacks per brigade has remained relatively constant – from about five to seven incidents per day.

Gates declined to predict how long U.S. troop levels in Iraq will stay this high. “We’ll just to have to wait and see the progress of these offensives,” he said.

Gates noted that decisions would be reached after reports on progress are made in September by Gen. David Petraeus, the top U.S. commander in Iraq, and Ryan Crocker, the U.S. ambassador there.