A car bomb blast in Afghanistan has killed five Americans, including three US soldiers and a young diplomat, while an American civilian died in a separate attack.

An Afghan doctor was also killed in Saturday's attack in Zabul province.

The US diplomat and other Americans were in a convoy of vehicles when the blast occurred, Secretary of State John Kerry said in a statement.

The soldiers and the diplomat died in the blast along with a civilian employee of the Defence Department and Afghan civilians, Kerry said. His statement gave no overall death toll.

The convoy was near a hospital and a NATO base at the time of the explosion.

"Our American officials and their Afghan colleagues were on their way to donate books to students in a school in Qalat, the province's capital, when they were struck by this despicable attack," Kerry said in his statement.

Provincial governor Mohammad Ashraf Nasery was in the convoy, but was unharmed.

"One doctor and one civilian were killed and two of my body guards have been injured," he told the AFP news agency.

Taliban claim

The Taliban claimed responsibility for the Zabul attack in a text message from Qari Yousuf Ahmadi, the group's spokesman.

He said a car bomb killed seven foreigners and wounded five others, though he later revised the toll to 13 foreigners killed and nine wounded.

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The Taliban routinely exaggerates casualty figures.

In a separate attack in Afghanistan's east, an American civilian working with the US government was killed during an armed attack, the NATO-led International Security Assistance Force said in a statement.

Saturday's attacks came as Martin Dempsey, chairman of US joint chiefs of staff, arrived in the country for a short visit to assess how much training Afghan troops need before US troops pull out as planned by the end of 2014.

The UN says civilians are being increasingly targeted in 2013.

A deadly Taliban assault in the country's west on Wednesday killed 44 people in a courtroom in Farah province.

In a statement posted online on Saturday, Ahmadi said the Taliban would continue to target Afghan judges and prosecutors.

"The Islamic Emirate, from today onwards, will keep a close watch over courthouses, all its personnel and all those who try to harm Mujahideen and will deal with them the same as the judges and prosecutors of Farah," he said.

Source: Agencies