Car bombs struck mostly Shiite targets in Baghdad on Wednesday, and the bodies of three Sunni professors and a student were found days after they were seized while leaving their campus in a Shiite part of the city. At least 43 people were reported killed across the country, including a US soldier. A mortar attack struck a predominantly Sunni neighborhood in northern Baghdad in apparent retaliation for the bombings, killing at least six people and wounding 20, police and hospital officials said. The violence underscored the extreme difficulties facing the capital's 6 million residents as they try to go about their daily business even as US and Iraqi forces gear up for a planned security sweep to clear the city of Sunni insurgents and Shiite militias who are blamed in many of the attacks. In preparation for the crackdown, Iraq indefinitely halted all flights to and from Syria and closed a border crossing with Iran, legislator Hassan al-Sunneid and an airport official said Wednesday. Maamoun Abdel-Hadi said he was standing with a friend near his car when a mortar shell fell nearby during the attack on the predominantly Sunni neighborhood of Azamiyah. The area was hit by nine mortar shells that damaged houses, shops and streets, killing six people and wounding 20, police and hospital officials said. "We fell on the ground but we were not hurt. I saw four wounded persons lying on the ground and screaming for help. We put them in the car and rushed them to the hospital," he said. "We are peaceful people who have nothing to do with any militias or armed groups. What is the guilt of innocent children, women and men who were walking in the street?" Jamal Ahmed mournfully examined his Mitsubishi car that had been burned in the attack. "Repairing my car will cost me a fortune, yet I thank God because I am safe and unhurt," he said. The mortar attack struck about 2 p.m., hours after parked car bombs struck Shiite targets elsewhere in the capital in what has become a common pattern in the violence plaguing Baghdad. One car bomb targeted a transit area near a busy market in central Baghdad where people can catch minibuses to predominantly Shiite neighborhoods, including the sprawling Sadr City slum. Four people were killed and 12 were wounded, police said. Another car packed with explosives blew up in the religiously mixed neighborhood of Maamoun in western Baghdad at about the same time, killing two civilians and wounding three others, police said, adding the target of the attack was not immediately known. A car bomb also struck a predominantly Shiite area in eastern Baghdad earlier Wednesday after the driver parked near a currency exchange office, then walked away, killing two people and wounding 10, police said. The shop's owner said the attacker appeared respectable when he came in to ask permission to park the car and exchange some money. "A seemingly normal person parked this car and told us that he would not be long," said the owner, who identified himself as Abu Talal. "When that person disappeared for more than 20 minutes, we tried to call the police but the car exploded as we were trying to do so." Shop owners often insist that motorists get permission before parking their cars due to the frequent car bombings in the capital. Insurgents have launched several bombings in the capital in recent weeks as they seek to maximize the number of people killed before US-Iraqi troops launch a neighborhood-by-neighorhood sweep of the city of 6 million. Iraqi authorities have promised to crack down on Sunni insurgents as well as Shiite militia violence that has spiraled since the Feb. 22, 2006, bombing of a Shiite shrine in Samarra. In all, 42 Iraqis were killed or found dead nationwide, including 20 bodies, many bearing signs of torture, apparently the latest victims of so-called sectarian death squads. A US soldier also was killed and another was wounded Wednesday in fighting in the mainly Sunni Salahuddin province north of Baghdad, while three others troops died in combat the day before west of the capital, the military said. The three professors and a student who was the son of one of the professors were abducted by gunmen on Sunday as they were leaving Nahrain University's law school in a predominantly Shiite neighborhood in northern Baghdad, according to the Ministry of Higher Education. Their bodies were found Wednesday in Baghdad's central morgue. Nahrain University, formerly called Saddam University, is one of the main educational institutions in Baghdad. It's main campus is in the Jadriyah area on the southeastern banks of the Tigris, but the law school is in Kazamiyah. Nobody claimed responsibility for the attack, but academics have been frequent victims of the rising sectarian violence, often targeted due to their relatively high public stature and vulnerability. Some professors also have been killed by students angered over poor grades or other grievances, or because of their past membership in the former ruling Baath Party - forcing many of them to flee the country to teach at universities abroad. Adnan Al-Janabi, the dean of the university, said the three professors were supervising many thesis students in addition to their daily lectures at the law school. "Our country has lost those who sacrificed their lives, they had the ability to emigrate abroad but their love of their country kept them from doing so," he told The Associated Press in a telephone interview. "We condemn and denounce such criminal, brutal acts against the Iraqi brainpower; we renew our demands for the government to bring the criminals to justice." Prime Minister Nouri al-Maliki, meanwhile, offered a new endorsement of US President George W. Bush's plan to augment American forces by 21,500 to help stem the violence in the country, but stressed that he considered it "a support to our Baghdad security plan," which would be an Iraqi-led operation. He also said they would only ask for extra troops as a last resort. "We agree this will be assessed by those in the field, the military commanders," he told CNN in an interview.