Military commanders in Iraq are developing a plan to move as many as 5,000 US troops with armored vehicles and tanks into the Baghdad in an effort to quell escalating violence, defense officials said Thursday. As part of the plan, Defense Secretary Donald H. Rumsfeld on Thursday extended the tours of some 3,500 members of the 172nd Stryker Brigade Combat Team. It was scheduled to be leaving now, but instead, most of its 3,900 troops will serve for up to four more months. It was unclear whether the Stryker troops, who are in northern Iraq, would be among those going to Baghdad. Under the plan to bolster security in Baghdad, US troops would be teamed with Iraqi police and army units and make virtually every operation in the capital city a joint effort, one military official said. Another said movement of some troops into Baghdad had already begun.