A car bomb exploded Sunday in a central Baghdad square, killing 10 people and wounding 25, in the deadliest attack on a day in which at least 18 others died in violence.

Still, a U.S. military spokesman on Sunday that the month-old American offensives in and around the capital were starting to have an effect, reducing violence and civilian deaths, though he did not cite figures.

The Bush administration is under increased pressure from critics in Congress who say the U.S. strategy in Iraq is not working, raising calls for a troop withdrawal. Proponents of a pull-out have pointed to the failure of Iraqi Prime Minister Nouri al-Maliki to enact political reforms considered vital to a long-term fall in violence.

The car bomb attack came in Hussein Square, a popular site of take-away restaurants in the central Baghdad district of Karradah near a bridge across the Tigris River, said a police official. The afternoon blast ripped through nearby stalls and shops, killing 10 and wounding 25, according to officials at the two hospitals where the victims were taken.

The dead included two women, and three women and five children were among those injured, said the hospital officials. All the officials spoke on condition of anonymity because they were not authorized to talk to the press. The area is near the offices of the country's largest Shiite party, the Supreme Islamic Council of Iraq.

"It was a big explosion and a fire followed," said one witness, the owner of a nearby mobile phone shop who would identify himself only by his first name, Haidar. "I rushed with others at site to see two burned corpses inside a car and wounded people."

In northern Iraq, gunmen ambushed a convoy of border guards, killing six of them along with a civilian, a border guard commander said. When reinforcements pulled in, another guard died in the clash, which took place in the Kani Khal area, 260 kilometers (160 miles) northeast of Baghdad. The commander said the Sunni extremist group Ansar al-Islam was believed to be behind the attack.

Elsewhere, shootings in the northern cities of Mosul and Kirkuk and several areas south of Baghdad killed eight people, according to police officers in the areas. Among there were the wife and son of the head of the city council in Aziziyah, 90 kilometers (50 miles) south of Baghdad, slain outside their home. The police officials and guard commander also spoke on condition of anonymity.

Rear Adm. Mark Fox told reporters that the sweeps in Baghdad, to the south and in the city of Baqouba to the northeast had brought a reduction in bloodshed in the capital -- particular in sectarian killings.

The offensives are "making a difference on the ground. We have seen a significant drop in the number of civilians murdered in Baghdad, the overall levels of sectarian violence has decreased," he said, without providing figures.

Recent weeks appear to have brought a decrease in dramatic car bomb attacks, though such bombings still occur nearly daily. But according to figures gathered by The Associated Press, the daily rate of bodies found dumped in Baghdad -- victims of sectarian slayings -- has risen slightly so far this month from June.

In the first 14 days of July, 301 bodies were found in Baghdad, or an average of nearly 22 a day, compared to 19 a day in June, when 563 bodies were found, according to AP figures, gathered from daily reports by Iraqi police.

Maj. Gen. Rick Lynch, commander of the U.S. offensive southeast of Baghdad, in the Salman Pak region, said the sweep had reduced attacks on civlians in that area by 20 percent and civilian deaths by 55 percent, compared to before it was launched in mid-June.

But Lynch said there was a critical need for more Iraqi security forces to secure the areas U.S. forces are clearing of Sunni extremists.

"There has to be a sustained security presence ... The issue is there has to be somebody who stays behind as a permanent security presence, and that won't be coalition (U.S) forces," he told AP.

Lynch said he expected it would take until the autumn to clear militants sanctuaries in the area, where 15,000 American troops are operating, and a "significant amount of time" thereafter to secure them. He said he expects to start handing over the areas to Iraqi security control in the spring.

Calls in the U.S. for a withdrawal have been fueled by a White House report released Thursday that showed little progress on the main benchmark political reforms that Washington is pressing the prime minister to carry out.

Al-Maliki on Saturday shrugged off U.S. doubts of his government's military and political progress, saying Iraqi forces are capable and American troops can leave "any time they want." He said the slow political progress was "natural" considering the security turmoil in the country.

One of al-Maliki's close advisers, Shiite lawmaker Hassan al-Suneid, bristled over the American pressure, telling AP that the U.S. is treating Iraq like "an experiment in an American laboratory (judging) whether we succeed or fail."

He sharply criticized U.S. military tactics, saying civilian deaths, raids on suspected Shiite militiamen in Baghdad's Sadr City slum and security walls erected around Baghdad's Sunni neighborhood of Azamiyah were "embarassing" al-Maliki's government.