Lt. Gen. David Petraeus is to arrive in Baghdad to take over for Gen. George Casey as the top U.S. commander in Iraq next week, a defense official said Monday.

Senate debate on a resolution opposing the U.S. troop buildup in Iraq could be getting under way about the same time.

The exact date Petraeus will assume command has not been set; Casey is coming to Washington to testify Thursday before a Senate committee considering his nomination to be Army chief of staff. Casey would then return to Baghdad to complete the transition with Petraeus a short time later, the official said. The official spoke on condition of anonymity because some details of the transition are still being planned.

The timing of Petraeus' takeover in Baghdad has become linked to the debate in Congress over proposed resolutions expressing opposition to President Bush's decision to send an additional 21,500 U.S. troops.

Defense Secretary Robert Gates said Friday that such a resolution would amount to undercutting Petraeus and emboldening the insurgents and other hostile forces.

Gates also said that at Petraeus' request the Pentagon is studying whether it could accelerate the deployment of at least some of the five additional Army brigades — totaling about 17,500 soldiers — that are to be sent to Baghdad between now and May. He said he would wait until Petraeus got established in Baghdad before he would consider the possibility of sending more troops than those announced by Bush on Jan. 10.

Bush, who chose Petraeus to replace Casey, said last week he wanted Petraeus to go to Baghdad as quickly as possible to begin implementing the new U.S. strategy for stabilizing the country. Petraeus faced no Senate opposition to his nomination, which was approved last week, but Casey may encounter some resistance to his selection as Army chief of staff, given the setbacks that occurred during his tenure in Iraq, which began in July 2004.

Casey would replace Gen. Peter Schoomaker, who is returning to civilian life. Schoomaker was brought out of retirement by former Defense Secretary Donald H. Rumsfeld to take the chief of staff job in August 2003.

In his new post, Petraeus will be promoted in rank to four-star general and report to the commander of U.S. Central Command. The holder of that post, Gen. John Abizaid, is due to retire in March; Navy Adm. William Fallon, who has been nominated to replace Abizaid, is scheduled to testify at his Senate confirmation hearing Tuesday.

Bush has not announced who he will nominate to replace Fallon as commander of U.S. Pacific Command. Among those believed to be a leading contender is Adm. Timothy Keating, who currently heads U.S. Northern Command.

Meanwhile, a small group of Iraq war veterans said they would embark Monday on a two-day trip to seven states to unveil a new television ad urging the Senate to stop Bush from implementing his troop buildup. The war veterans are members of VoteVets.org, which describes itself as a pro-military, bipartisan group that supports the destruction of terrorist networks.