A rocket landed Monday night near coalition headquarters in the U.S.-controlled area of Baghdad known as the "green zone" but caused no injuries or damage, the U.S. military said.

The rocket fell in an open parking lot near the Republican Palace used by top U.S. officials in Iraq, a coalition official said on condition of anonymity. The U.S. command said there were no casualties or damage to equipment.

That explosion was preceded by the sounds of two smaller blasts, but their origin was unclear.

After the blasts, sirens went off in the green zone and a recorded message broadcast over loud speakers urged people to "take cover."

The rocket exploded near the helicopter landing zone for the palace used by America's top civilian official in Iraq, L. Paul Bremer (search), and senior coalition staff, the official said.

The attack occurred three days after a U.N. security assessment team arrived in Baghdad to determine whether it is safe for the United Nations to return its international staff to Iraq.

Secretary-General Kofi Annan (search) withdrew international staff from Baghdad last year following two vehicle bombings against agency headquarters, including the Aug. 19 attack that killed 22 people, including top U.N. envoy Sergio Vieira de Mello (search).