BAGHDAD, Iraq (AP) – A suicide car bomber killed 17 Shiites at a teeming Sadr City market Wednesday, while gunmen in a predominantly Sunni neighborhood of Baghdad shot up a convoy of democracy workers in an ambush that took the lives of an American woman and three bodyguards.

The attack on the marketplace came one day after car bombings killed scores of university students just two miles away, indicating that al-Qaida-linked fighters are bent on a surge of bloodshed as U.S. and Iraqi forces gear up for a fresh neighborhood-by-neighborhood security sweep through the capital.

Although nobody claimed responsibility for either day’s car bombings, such attacks are the hallmark of Sunni militants, who appear to be taking advantage of a waiting period before the security crackdown to step up attacks on Shiites. There had been a relative lull in Baghdad violence since the first of the year.

An Iraqi army officer said the attack on the Western convoy took place in Yarmouk, a predominantly Sunni neighborhood in western Baghdad.

The three-car convoy belonged to the Washington-based National Democratic Institute, according to Les Campbell, the not-for-profit group’s Middle East director. He said the four dead included an American woman along with three security contractors – a Hungarian, a Croatian and an Iraqi. Two others were wounded, one seriously, Campbell said by telephone from Washington.

Their names were withheld until their families could be notified.

“It appeared to be an attack with fairly heavy weapons, we don’t know what kind,” Campbell said. “We have some information that a firefight ensued. Our security company responded to the attack.”

Campbell said the ambush took place at midday as the group returned from a program elsewhere in Baghdad.

Few foreigners and even fewer women have been caught up in Iraq’s recent wave of violence as many Western groups have left and those who remain have tightened security and curtailed their movements after a series of kidnappings and beheadings.

The last known American female civilian to be killed was Marla Ruzicka, a 28-year-old rights activist from California who died in a car bombing in April 2005.

Prime Minister Nouri al-Maliki did not give a start date when he announced plans for a new drive to tame the violent capital – the third attempt since he took office May 20.

But U.S. and Iraqi reinforcements have started to arrive in Baghdad, and it was expected to begin in about two weeks.

The marketplace explosion took place just before 4 p.m. near a popular commercial area in Sadr City, a sprawling Shiite district of some 2.5 million people in eastern Baghdad.

The blast shattered the windows of nearby shops and restaurants, and blood pooled in the street. Angry Iraqis surrounded the charred mass of twisted metal, all that was left of the explosives-packed car. They tipped the remains on its side and picked off pieces of blackened upholstery.

At least 17 people were killed and 33 people were wounded, police said.

In many parts of the capital, streets were crowded with cars and minivans carrying wooden caskets of the victims from Tuesday’s car bombings, which killed at least 70 people and wounded more than 130 at Al-Mustansiriya University. Many vehicles were headed to the holy city of Najaf where Shiites prefer to bury their dead. Other victims were taken to a Sunni cemetery in central Baghdad. The students were from all the country’s religious sects.

Hussein Mohammed, a lecturer in the university’s French language department, said classes were canceled for two days while workers cleared the debris. “We are trying to heal our wounds and start again,” he said.

The Iraqi parliament stood for a moment of silence and lawmakers and students demanded stepped-up security for schools and universities.

Al-Maliki announced the new security drive Jan. 6, four days before President Bush detailed his version of the plan with an announcement that he was sending 21,500 more U.S. troops to Iraq.

There have been concerns that insurgents would just slip out of the capital to wait out the offensive. Some appear to have left, given the spike in violence in northern Iraq, where Sunni militants have retreated in the past.

State television reported that at least 100 insurgents were killed Wednesday in clashes with Iraqi troops in a predominantly Sunni region northeast of Baghdad. Troops captured dozens of insurgents and seized large amounts of ammunition, the state-run Iraqiya channel said, quoting police. The fighting reportedly took place near the district of Balad Ruz, 45 miles northeast of the capital.

In oil-rich Kirkuk, 180 miles north of Baghdad, a suicide car bombing killed 10 people and wounded dozens at a police checkpoint.

In all, police reported 70 people killed or found dead in Iraq on Wednesday. They included 31 bullet-riddled bodies that turned up in Baghdad showing signs of torture, victims of apparent death squads largely run by Shiite militias like the Mahdi Army, which has its stronghold in Sadr City.

The U.S. military also said two more American soldiers died – one Wednesday after suffering wounds during an operation in the Sunni stronghold of Anbar province west of Baghdad and another who died there Monday.

Al-Maliki, meanwhile, met with the ambassadors of several countries, including the United States, to shore up support for his planned security operation. He pledged to act equally against all gunmen, regardless of sect, his spokesman said. The Shiite prime minister is under heavy criticism over his interference in U.S. attempts to confront Shiite militias during two failed attempts to bring calm to Baghdad.

“We want the international community to understand that the Baghdad security plan is targeting all the outlaws, it does not target a specific group or specific area, rather it targets all Baghdad,” said Ali al-Dabbagh, the spokesman.

Throughout the Middle East, Arab leaders were deeply skeptical of the U.S. plan for Iraq, a day after Secretary of State Condoleezza Rice tried to sell it to them. Kuwait’s emir told Rice that America should work with Iran and Syria, officials said – a move Bush has rejected.

The National Democratic Institute, the group whose convoy was attacked Wednesday, supports democratic processes and institutions worldwide. Its staffers in Baghdad run training programs in democracy and political participation, as well as women’s rights. The group has had staffers in Iraq since June 2003, though Campbell would not specify how many, for security reasons.

Kenneth Wollack, president of the organization, said in his Washington office that “this is a tragedy that has hit individuals that have been dedicated to the democratic future of Iraq.”

The American woman was the first full-time worker for the group to be killed in Iraq. A security contractor for the organization was killed in March 2004.

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Associated Press writer Lauren Frayer in Baghdad contributed to this report.