WASHINGTON -- Top Pentagon officials yesterday signaled that further troop reductions in Iraq are likely and that more American forces could be sent to Afghanistan sooner than previously planned.

Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said that on his recent trip to Iraq he found security conditions, though tenuous, had improved more than he expected.

"If these trends continue, I expect to be able early this fall to recommend to [Defense Secretary Robert Gates] and the president further troop reductions," he said.

But in a sign of the fragility of security gains in Iraq, a car bomb detonated yesterday in a market in northern Iraq and killed 18 people, including seven children.

Meanwhile, the insurgency in Afghanistan is growing. Gates said a quicker deployment of troops is being considered but that no final decisions or recommendations have been made.

"I think that we are clearly working very hard to see if there are opportunities to send additional forces sooner rather than later," Gates told reporters.

His comments suggested an acceleration in what had been plans to shift forces there early next year. And they came as the political discourse on Afghanistan as a key military priority escalated on Capitol Hill and the presidential campaign trail.

The officials' comments come just days after a forward operating post in Afghanistan was overrun by al-Qaida and Taliban militants, killing nine Americans and several Afghan soldiers. Gates and other officials have warned for weeks of the growing threat, particularly along the Afghan-Pakistan border.

In Iraq, meanwhile, the military buildup that began more than 18 months ago has ended. In recent days, the 3rd Infantry Division's 2nd Brigade, the last of the five additional combat brigades sent in by President Bush last year, left the country.

Its departure marks the end of what the Pentagon calls the surge. And it starts the 45-day evaluation period that Gen. David Petraeus, the top U.S. commander in Iraq, told Congress he would need to assess the security situation and determine how many more troops he could send home.

Yesterday's car bombing in Tal Afar, Iraq, was a deadly reminder that militants still can cause casualties despite security improvements.

In addition to the 18 killed, 90 people were injured in the blast at a popular outdoor market, said a police official, who insisted on anonymity because he was not authorized to speak to the news media.

The city, a one-time stronghold of Sunni insurgents about 260 miles northwest of Baghdad, was targeted in offensives by U.S. and Iraqi troops that prompted American leaders to describe it as a success story in the effort to stabilize Iraq. But sporadic attacks continue.

Also in the north, a car bomb killed two civilians in Mosul, police reported. The two attacks came a day after suicide bombers killed at least 28 people in Baqouba, northeast of Baghdad.

South of Baghdad yesterday, the formal transfer of security in Qadisiyah province reflected a drop in violence and marked another success for Iraq's increasingly assertive government, which seeks a timeline from the United States for the withdrawal of American forces.

Qadisiyah, a mostly Shiite region, was the 10th of Iraq's 18 provinces to return to Iraqi authority, with U.S. and Polish troops relinquishing control at a military ceremony.

"This is further evidence of our goal to have security control in the whole of Iraq by the end of 2008," said Mouwafak al-Rubaie, Iraq's national security adviser.

In a statement, U.S. Ambassador Ryan C. Crocker and Petraeus said Iraqi security forces in Qadisiyah had been operating independently the past two months.

"We will assist as requested," the statement said, adding that Iraqi provincial and military leadership would have to create long-term security that can lead to economic development.