WASHINGTON - Anticipating a new burst of insurgent     (see Editorial, Gasttoday) violence, the Pentagon plans to expand the U.S. force in Iraq to improve security for a planned October referendum and a December election.

Although much public attention has been focused recently on the prospect of reducing U.S. forces next spring and summer, defense officials foresee the likelihood of first increasing troop levels.

Lawrence Di Rita, spokesman for Defense Secretary Donald H. Rumsfeld, noted Monday that troop levels were raised last January during Iraq's first elections, and then returned to the current level of about 138,000 several weeks later.

"It's perfectly plausible to assume we'll do the same thing for this election," he said, while stressing that no decisions had been made.

Di Rita said he did not know how many extra troops might be needed during the referendum and election period.

Other officials have said that once the election period has passed and the troop total recedes to the 138,000 level, a further reduction in the range of 20,000 to 30,000 is possible next spring and summer. That could change, however, if the insurgency intensifies or an insufficient number of U.S.-trained Iraqi security forces prove themselves battle ready.

Last January the U.S. troop level rose as high as 160,000. This was accomplished mainly by overlapping some units arriving in Iraq to begin a one-year tour with those who were ending their yearlong tours. In at least one case an Army brigade was kept a little longer than its scheduled 12 months in Iraq, and Di Rita said he could not rule out this happening again this fall, although the intention is to avoid tours longer than 12 months.

"The units that are there have been told to expect that," he said. "It's possible that your planned rotation dates back to the U.S. will be affected by the need to keep a higher level for a longer period of time. They understand that."

Di Rita said commanders may also ask for volunteers to serve extended tours.

Another possibility is that some U.S.-based troops will be sent to Iraq to augment the force during the election period. One unit called upon most frequently for that kind of duty is the 82nd Airborne Division, which currently is deploying a battalion to Afghanistan to bolster security in advance of Sept. 18 elections there.

Di Rita said no elements of the 82nd had been alerted to prepare for similar duty in Iraq this fall.

U.S. commanders predict a need for extra troops this fall in Iraq because the insurgents have tended to intensify their attacks when key political milestones approached. If a draft constitution is ready by Aug. 15, as intended, then a national referendum on that charter is to be held Oct. 15, followed by December elections based on the constitution.

Rumsfeld recently mentioned his expectation that insurgent violence will increase in the weeks ahead.

"We can reasonably expect - both in Afghanistan and in Iraq - to see an increase of violence as they continue to move towards their political goals: in the case of Afghanistan, September 18th - the provincial and the parliamentary elections - and in the case of Iraq, the referendum on October 15th for the constitution," he said July 20.

Among the Army units scheduled to deploy to Iraq in coming months is the 101st Airborne Division, which was part of the original invasion force in 2003 and returned home early in 2004, as well as the 4th Infantry Division, which arrived in Iraq shortly after the fall of Baghdad in April 2003. Those two divisions have since been reorganized and now have four combat brigades each, rather than three each.