At least seven people have been killed and dozens more wounded in a suicide bombing in the capital city of Afghanistan's Helmand province, according to officials.

The bomber detonated an explosives-packed car next to an Afghan army vehicle as soldiers arrived at a bank in Lashkar Gah to collect their pay, Omar Zwak, spokesman for the Helmand governor, told Reuters news agency.

Among the dead were four civilians and three soldiers, Zwak said.

The Taliban later claimed responsibility for the attack, which also wounded 16 civilians and four soldiers.

Qari Yusuf Ahmadi, a Taliban spokesman, said on Twitter that 39 soldiers and officers had been killed and wounded "in the martyrdom attack".

Different sources cited conflicting death tolls.

"A suicide car bomber killed six people, including five soldiers, and 21 others were wounded," Agha Noor Kentoz, Helmand police chief, told AFP news agency.

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Hundreds of international troops are stationed in Helmand as part of a NATO-led effort to train and support Afghan security forces.

At least one US special forces soldier was wounded in fighting there this week.

To the north of Lashkar Gah, a local official said that an American military air strike killed a number of civilians in a recent bombing in Sangin district.

The allegation has not been independently verified.

Captain Bill Salvin, a US military spokesman, said US fighter jets had conducted strikes in Sangin in the past few weeks.

While US forces had "no evidence that civilians were killed in these strikes", Salvin said the command would investigate the claims.

"We take every precaution to prevent and mitigate civilian casualties and we take every allegation seriously."