"Two US service members and one US civilian contractor received minor injuries today when a suicide car bomb exploded outside the main gates of the Lashkar Gah Provincial Reconstruction Team compound in Helmand," the coalition said in a statement.
"The suicide bomber was killed in the blast. A nearby pick-up truck and the bomber's vehicle were destroyed," it said.
Conflicting reports
Earlier provincial police chief Abdul Rahman Saber said the attack was against British troops and that first reports said some may have been wounded.
"At least one British military vehicle could be seen damaged," he said.
And according to the provincial governor Mohammad Daud the blast appeared to have struck a vehicle of US-based security firm DynCorp.
"A DynCorp vehicle was passing by when the explosion occurred. According to the latest information I obtained, no one has been hurt," Mr Daud said.
Mr Daud blamed the attack on "Al-Qaeda terrorists."
Home to troops
The base in Lashkar Gah is home to US troops and about 150 British soldiers who are preparing for the arrival of more than 3,000 troops in the coming weeks.
In the past week there has been a string of suicide attacks in the country, most of them killing only the attackers.
Helmand has seen regular attacks against coalition soldiers who have been in Afghanistan since late 2001.
The province is the main producer of Afghanistan's crop of illegal opium, which makes up around 90 percent of the world's supply.