Washington ? About 21,000 American soldiers in Iraq who were to return this month to their home bases in Louisiana and Germany will have their tours extended at least three months to help combat the surge in anti-occupation violence, defense officials said Wednesday.

The decision, which has not been announced publicly, breaks the Army’s promise to soldiers and their families that assignments in Iraq would be limited to 12 months. The affected soldiers already have been in Iraq for a year.

In addition, about 1,000 soldiers in transportation units based in Kuwait will be extended beyond one year, a senior defense official said. Most of them are in the National Guard or Reserve. They are deemed critical to re-supplying the troops based in Iraq.

Welcome-home ceremonies at Fort Polk, La., scheduled for this month, have been canceled. In Baumholder, Germany, some soldiers’ families have stopped marking the days off the calendar.

The top U.S. commander for the Middle East, Gen. John Abizaid, decided that the increase in violence was so threatening that he needed to have the extra firepower, officials say.

Defense Secretary Donald H. Rumsfeld was fine-tuning the new plan Wednesday; his spokesmen declined to discuss details. They said Rumsfeld might make it public today.

The tour extensions come at a particularly delicate moment. At least 87 troops have been killed in April, the deadliest month since they set foot in Iraq in March 2003. The number of wounded also has skyrocketed.

Of the estimated 21,000 soldiers affected by the extension in Iraq, about 18,000 are in the 1st Armored Division. About 2,800 are with the 2nd Armored Cavalry Regiment.

The advantage of keeping soldiers of the 1st Armored and the 2nd Armored Cavalry in Iraq for an extra three months — rather than bringing in an equivalent number from elsewhere — is that these soldiers have unmatched combat experience in Iraq.

The Army is so stretched by its commitments in Iraq, Afghanistan, the Balkans and elsewhere that it has few, if any, forces immediately available to substitute in Iraq for the 1st Armored or 2nd Armored Cavalry.

Also, these units have been heavily involved in one of the most important U.S. military missions there: training thousands of Iraqi security forces. Those Iraqi army and civil defense corps members are central to the Pentagon’s plan for eventually turning over military control to the Iraqis and pulling out U.S. troops.

Abizaid had planned, as part of the current rotation of fresh forces into Iraq, to reduce the U.S. troop presence from about 135,000 to about 115,000.

But the surge this month in anti-occupation violence in restive areas in and around Baghdad and in the south has forced Abizaid to change course.

He indicated on Tuesday that he needed more forces than originally planned. He would not tell reporters exactly how many or where he would get them.