Nearly 20 people were also wounded in the explosion which occurred near the Canal Hotel on Monday morning, a US soldier said.
Witnesses told Reuters there was at least one person killed in the blast, which US military sources confirmed had been caused by a resistance fighter driving a car bomb.
The attack came a day after two American soldiers were killed in a mortar attack and a third died from a roadside bomb blast in the latest strikes against US occupation forces in Iraq.
It also came on the eve of the annual United Nations General Assembly in New York, which an Iraqi delegation had been due to attend.
One of those due to represent Iraq at the assembly, Iraqi Governing Council member Aqila al-Hashemi, was critically wounded on Saturday in an attack in Baghdad.
The blast happened at about 8.10am (0410 GMT), 250 metres (yards) from the United Nations headquarters, and destroyed at least six other vehicles.
The resistance fighter drove into the UN car park and was stopped by an Iraqi security guard before reaching the parking lot's main checkpoint.
It is thought the attacker had changed his target because of the security in place around the UN building.
In August an attack on the United Nations headquarters in Baghdad killed Sergio Vieira de Mello, the UN's top envoy to Iraq, among more than 20 people who died in the bombing.
Rising violence has put US President George Bush under pressure at home and Washington is urging other countries to send troops to Iraq to help keep the peace after the war that toppled Saddam Hussein in April.
Since Bush declared major combat over on 1 May, 79 US soldiers have been killed in hostile incidents in Iraq.