Gunmen in suicide vests have detonated a truck bomb outside a US consulate in western Afghanistan, killing at least one person.

The US State Department said there were no American casualties in the attack, which took place in the city of Herat, and the incident is now over.

The US embassy in Kabul confirmed on Twitter that all consulate staff were safe and accounted for.

The front gate of the consulate was "extensively" damaged, said Marie Harf, US State Department deputy spokeswoman.

Afghan officials said at least one person had been killed and 18 others wounded. Four Afghan policemen were among the wounded, said a hospital spokesman.

The attack began at 5.30am local time when gunmen in suicide vests drove up to the front gate in a truck, opened fire and then detonated the truck bomb.

"It appears American and contract security personnel addressed any attackers who managed to enter the compound," said Ms Harf.

Herat, near the Iranian border, is considered one of the least volatile cities in Afghanistan.