The British defence ministry later confirmed that two of its troops, who were serving in the Nato-led International Security Assistance Force (Isaf), also died in the bombing.
There was no immediate claim of responsibility for the attack.
"A suicide bomber on motorcycle headed from a street to the main road where forces were patrolling and then he fell to the ground," a witness said.
"People gathered to help him up. When the Isaf troops arrived, he detonated himself," he said.
The latest attack is similar in execution to other assaults carried out by Taliban fighters in Helmand.
The bombing was the deadliest such attack since a suicide bomber killed 25 policemen in the southern province of Uruzgan in February, authorities said.
The Taliban, who were in power from between 1996 and 2001, warned last week that they would step up attacks on troops and government officials.
About 70,000 troops are in Afghanistan to help the Western-backed government battle fighters linked to the Taliban.
About 8,300 of the international soldiers serving in the country are British.