Six explosions boomed across central Baghdad (search) on Sunday, sending plumes of smoke into the air. One blast hit a truck traveling on a downtown street, setting it ablaze and causing casualties, officials said.

Flames and smoke leapt from the destroyed truck as firefighters tried to put out the fire. The blast, apparently from a rocket or mortar, collapsed part of a wall of a nearby house, and there were blood stains on the street.

Shrapnel also smashed the windows of a nearby car, the seat of which was covered blood. Police on the scene said there were causalties but could not say how many.

Police cordoned off the area, and one man looking for a relative pleaded to be allowed to pass. "My Brother! My Brother," he yelled.

The explosions came in two rounds, and the cause was not known — though insurgents frequently fire volleys of mortars (search) or rockets in the city.

Four explosions hit in quick succession. Several minutes later two more blasts shook downtown, and the crack of gunfire rang out.

One of the blasts hit a bank of the Tigris River (search) bordering the Green Zone (search), the fortified enclave housing the Iraqi government's offices and the U.S. Embassy.

After the second set of explosions, a siren sounded in the Green Zone.