Assailants gunned down the chief of Saddam Hussein's tribe in the ousted leader's hometown of Tikrit a few weeks after he publicly disavowed Saddam. Although the motive was unclear, Abdullah Mahmoud al-Khattab had many enemies, the regional governor said.
Elsewhere in Iraq, US troops shot and killed four people at checkpoints and a mosque explosion killed 10 people in Fallujah, further stirring anti-American sentiment in a town where Saddam and his Baath Party still enjoy support.
Two attacks against American forces on Tuesday wounded at least six soldiers.
In Baghdad, the top US official in Iraq, Paul Bremer, said the US-led provisional authority was "well on track to establish an Iraqi interim administration by mid-July."
"Day by day, conditions in Iraq continue to improve," he said.
Despite his reassurances, a burgeoning insurgency has seen several attacks on US troops every day, leading some to worry about the possibility of a Vietnam-style political and military quagmire.
A delegation of nine US senators on a three-day tour of Iraq expressed confidence on Tuesday in the US mission, but acknowledged that risks remain.
In Tikrit, Abdullah Mahmoud al-Khattab, who was leader of Saddam's Bani al-Nasiri tribe, was shot and killed on Sunday afternoon while he rode in his car.
The killing highlighted the shifting alliances that have characterized Iraq as the country emerges from 35 years of brutal, one-man rule. Even those eager to distance themselves from Saddam often pay dearly for their past links to him.
Saddam still enjoys a degree of popularity in Tikrit, where he built roads and schools and soccer fields.
Most other Iraqis express disdain for Saddam, yet anti-US forces have persisted in stepping up attacks on occupation forces in recent days.
On Tuesday, assailants traveling in a vehicle in central Baghdad fired a rocket-propelled grenade at a US military vehicle, wounding three soldiers. Another grenade slammed into a US truck on a road south of Baghdad, injuring three soldiers.
In western Baghdad, US troops shot and killed two people when their car didn't stop at a checkpoint, witnesses said. Later, two civilians were shot and killed at another checkpoint, one by soldiers who feared he was an insurgent and another by a stray bullet, witnesses said.
The increasing attacks have killed more than 22 US soldiers and wounded dozens more since major combat was declared over on May 1, and many troops have become quicker to pull their guns.
In Fallujah, a blast in a cinderblock building in the courtyard of the al-Hassan mosque killed 10 Iraqis and wounded four late on Monday, said Colonel Guy Shields, spokesman for the US military in Baghdad. Iraqis insisted the blast was caused by a US missile -- an account the military denied.
Fallujah, 55km west of Baghdad, has been a hotbed of anti-US activity and scene of several confrontations involving US troops.
Meanwhile, a weekend explosion at an ammunitions depot killed at least 15 people and injured at least four near Hadithah, 240km northwest of Baghdad.
Amid the renewed violence in Iraq, the US Defense Department is trying to figure out how many postwar troops should stay and when it can bring home some of the longest-serving.