Violence in Afghanistan has surged over the last year to its worst level since 2001, with Taliban militants launching more suicide and roadside bomb attacks in an attempt to show that the Afghan government and its Western backers cannot bring security.
The suicide bomber detonated his explosives next to an army vehicle in the Mir Awad area of Herat province, the Defence Ministry said in a statement.
Separately, US-led coalition forces together with Afghan officials were investigating claims on Wednesday they had killed civilians during an operation targeting a Taliban network in Kapisa province, northeast of Kabul, the US military said.
The issue of civilian casualties is sensitive in Afghanistan and has eroded support for international forces. President Hamid Karzai said on Tuesday the deaths were a main source of instability in the country.
The US military said it killed 18 militants and one Taliban commander in Tuesday's operation, a claim it also says is supported by Afghan officials. But Afghan news agency, Pajhwok, reported residents as saying 25 civilians had been killed.
"We take reports of civilian casualties very seriously," said US military spokesman Colonel Greg Julian in a statement. "Our investigation into recent operations in Kapisa will weigh all inputs to determine the truth," he said.
In another incident, Afghan soldiers killed two insurgents and detained one suspected militant during an operation in the southern province of Kandahar on Tuesday, the Defence Ministry said in a statement.