WASHINGTON - The Pentagon announced yesterday that 43,000 National Guard and Reserve support troops will begin replacing U.S. forces in Iraq early next year and that most of those part-time soldiers will be called to duty within 30 days.

One unit from Maryland - an Army National Guard engineering outfit from the Baltimore area - is expected to be part of the call-up, though officials declined to identify the unit until its members have been notified. The 150 soldiers in the unit are receiving an alert order, officials said, meaning that they will likely be called to active duty.

Four other Maryland Army National Guard units, including about 600 soldiers, are already serving in Iraq and Afghanistan, said Maj. Charles Kohler, a spokesman for the Maryland National Guard.

The Pentagon's troop rotation plan comes at a time of anxiety about the long deployments being endured by reservists and about the Bush administration's failure to persuade other nations to contribute more troops to the coalition force in Iraq.

Overall, the U.S. force in Iraq is projected to decline next year by more than 20 percent from its current level. That decrease, though, is based on the assumption of improved security, additional foreign troops and an expanded Iraqi security force.

A call to duty

The reservists who will be called up from around the nation are from support units, more than 300 in all, from virtually every state and territory, officials said, though few details were released as of last night. The soldiers include truck drivers and intelligence specialists, as well as military police. The Army National Guard will supply 37,000 of the reservists; the remaining 6,000 will come from the Marine Corps Reserve.

One official said 28,000 of the troops would be called to duty within a month. The rest are to be activated in two increments, one on Dec. 15 and another after the first of the year.

In addition to the 43,000 reservists for Iraq, the Pentagon said it alerted 3,700 National Guard and Reserve troops who might be mobilized for Afghanistan.

Before yesterday's announcement, there were 154,603 members of the National Guard and Reserve on active duty.

"In this force rotation, we've tried to give people the longest notice possible so that they, their families, their employers will have some time to prepare," Defense Secretary Donald H. Rumsfeld said at a Pentagon news briefing.

Rumsfeld said the Army troops would be called up for a maximum of 18 months, with up to 12 months of that time in Iraq. The Marine mobilizations will be for 12 months, with up to seven months in Iraq.

The troop rotation plan includes 85,000 soldiers and Marines from active-duty units, including elements of the 1st Marine Division from Camp Pendleton, Calif., the 25th Infantry Division from Hawaii, the 1st Cavalry Division from Fort Hood, Texas, and the 1st Infantry Division, based in Germany.

Besides those active units, there are three Army National Guard combat brigades from

North Carolina, Washington and Arkansas, totaling about 15,000 soldiers, who will be sent to Iraq early next year.

It would be the second trip to Iraq for the 1st Marine Division, which played a key role in the three-week war, driving from Kuwait into Baghdad. But officials note that, with transfers to other units, many of the Marines who fought earlier won't be among those returning. About 20,000 Marines are expected to be part of the package.

Those active units will replace units now in Iraq, including the 101st Airborne Division, the 4th Infantry Division and the 1st Armored Division. The Pentagon had hoped to replace the 101st with another multinational division beyond the two there now. But the Bush administration has failed to persuade other nations to contribute to another division. There are now about 24,000 non-American forces in Iraq, about half of them British.

Marine Gen. Peter Pace, vice chairman of the Joint Chiefs of Staff, said the number of U.S. troops could be cut to about 100,000 by May, from the current 130,000. But officials cautioned that the reduction would depend on an improved security situation, more contributions of troops from other countries and an increase in the number of Iraqi security forces, which now stand at 115,000. Officials hope to increase the number of Iraqi forces to 170,000 by May.

'Ask for more help'