BAGHDAD (AP) – Nearly 4,000 American soldiers have arrived in the capital to strengthen the 12-week crackdown aimed at quelling sectarian violence, the American military said Wednesday, as bombings and shootings killed 12 people across the country.

The developments came on the eve of an international conference on Iraq being held in the Egyptian Red Sea resort of Sharm el-Sheik at which the American administration is expected to press hard for countries to forgive billions of dollars in Iraqi debt to help the Shiite-led government.

The American military said Wednesday that the fourth of five brigades being sent to help Iraqi security forces as part of the crackdown had arrived this week.

The 4th Brigade, 2nd Stryker Brigade Combat Team from Fort Lewis, Wash., which includes about 3,700 soldiers, will be deployed in the Baghdad area and in northern Iraq, the military said. Officials want the rest in place by June, for a total in Iraq of 160,000.

American military spokesman Rear Admiral Mark Fox said Wednesday that Iraqi and American forces now have 57 joint security stations and combat outposts in the Baghdad area and that “while the security situation remains exceedingly challenging, we’ve seen some encouraging signs of progress.”

“We continue to see a reduced total number of sectarian incidents in comparison to before the Baghdad security operation, including murders and kidnappings,” Admiral Fox told reporters in Baghdad. But he said car bomb attacks have increased, including some with very high casualties.

When complete, the Baghdad security operation will include about 28,000 additional U.S. forces, including 20,500 combat soldiers and about 8,000 service members involved in support services such intelligence, military police and logistics.

Brigadier General Qassim al-Moussawi, an Iraqi military spokesman, said most of the crackdown’s operations were taking place in volatile areas outside Baghdad, including the Sunni cities of Mahmoudiyah and Madain.

General al-Moussawi said insurgent operations had dropped significantly in Baghdad as the groups had fled to other areas.

“Next week will witness more military operations in both halves of Baghdad,” he said, referring to the two sides of the Tigris River that divides Baghdad. “Almost all our military operations are now taking place on Baghdad’s outskirts.”

The security efforts come as President Bush is engaged in a fierce debate with the Democratic-led Congress over the war in Iraq. Mr. Bush vetoed legislation to pull American troops out of Iraq Tuesday night in a historic showdown with Congress over whether the unpopular and costly war should end or escalate.

The measure would would require the first American combat troops to be withdrawn by Oct. 1 with a goal of a complete pullout six months later.

“This is a prescription for chaos and confusion and we must not impose it on our troops,” Mr. Bush said in a nationally broadcast statement from the White House. He said the bill would “mandate a rigid and artificial deadline” for troop pullouts, and “it makes no sense to tell the enemy when you plan to start withdrawing.”

Democrats accused Bush of ignoring Americans’ desire to stop the war, which has claimed the lives of more than 3,350 members of the military.

Ismail Qassim, a 41-year-old Shiite electricity ministry employee in Baghdad, welcomed the move.

“In spite of all the problems Iraq is facing because of the American presence, there is some need for them at least for one more year because of the sectarian strife in Iraq and corruption in the security services,” he said.

But Sameer Hussein, a 22-year-old Sunni college student in Baghdad, said he wanted the American forces to withdraw but didn’t think they ever would.

“Even if they will withdraw they will leave permanent military bases in Iraq and that is something Iraqi people will reject,” he said.

A senior Interior Ministry official, meanwhile, said officials were trying to gain custody of Abu Ayyub al-Masri’s body amid widespread skepticism over claims that the leader of Al Qaeda in Iraq had been killed.

Major General Hussein Kamal declined to comment further, but a police official in Anbar province said al-Masri died when his explosives belt detonated during fighting but security forces could not retrieve the body because it was in a part of the desert controlled by the terror group.

American authorities urged caution about the reports, saying they had not been confirmed and warning that even if the claim were true, the death of the shadowy Egyptian militant likely would not spell the end of the terror movement in Iraq.