A string of bombings in a southern oil city killed 19 people Thursday evening and injured dozens more, a grim sign of the security challenges Iraq will face after American troops go home.

The U.S. military is drawing down its troops ahead of an end-of-December deadline to have all American forces out of the country. Incidents like Thursday's triple bombing in a city seen as key to Iraq's economic development show the dangerous prospects awaiting Iraqis next year.

Three bombs went off in a popular open-air market in Basra, police and health officials said.

The third bomb exploded a few minutes after Iraqi army and police forces arrived on the scene in response to the earlier blasts, officials said. The third blast caused all the fatalities and almost all of the injuries, the officials said.

Among the dead and wounded were many policemen and Iraqi army soldiers.

The police officials spoke on condition of anonymity because they were not authorized to speak to reporters.

Kamal Ali was working at a clothing shop across the street when the blasts went off. He said after the first explosion, bystanders rushed to help the victims. When another blast went off about five minutes later, the terrified people ran to escape.

Then police and soldiers rushed to the scene before the third and most deadly bomb went off.

"Most of the casualties are police and Iraqi troops who rushed to help the victims and cordoned off the scene. They sacrificed their lives for the poor people," Ali said.

The head of the Basra provincial council, Ahmed al-Sulaiti, confirmed the incident.

"We can't blame the security forces for this act. They were the people most hurt," he said by telephone from Basra.

Basra is about 340 miles southeast of Baghdad It is considered the center of Iraq's burgeoning oil sector.

Many foreign oil companies have offices there. The country is relying on foreign companies to bring the money and expertise needed to develop Iraq's vast oil sector, which has been ravaged by war, sanctions and neglect.

There was no immediate claim of responsibility for the bombings, and it was not clear whether it was the work of Sunni extremists like Al Qaeda, or by Shiite militias. Sunni militants often stagger their blasts in order to cause the most carnage, and they often target security officials, whom they see as propping up the Shiite-led government.

The area where the blasts occurred is also a stronghold for Shiite militia members, who have been known to use violence as they jockey for power and control.