Six NATO personnel have been killed in two separate attacks in Afghanistan, making it the deadliest day for foreign forces this year.

Three American soldiers and two civilian workers, one from the US, were killed when a Taliban car bomb struck a NATO convoy in the southern province of Zabul, a US security source said.

American and Romanian troops are based in the province, according to the International Security Assistance Force (ISAF) website, but the force did not disclose the victims' nationalities in line with its policy.

An unknown number of other Afghan civilians were also killed.

US secretary of state John Kerry said the group was travelling to donate books to students at a school in Qalat, the capital of southern Zabul province, when they were killed in the "despicable attack".

Among the dead was an American diplomat who Mr Kerry met in Kabul last week.

He described her as "smart, capable, eager to serve".

"(She was) deeply committed to our country and the difference she was making for the Afghan people," he said.

The Zabul blast left four state department staffers injured, one critically, according to Mr Kerry.

Zabul provincial governor Ashraf Naseri said he was travelling to the city of Qalat when he heard the blast.

"One doctor and one civilian were killed and two of my bodyguards have been injured," he said.

An American citizen also died in what authorities are calling "an insurgent attack" in eastern Afghanistan.

It was the deadliest day for the coalition since July 8 last year, when seven soldiers died in separate attacks in southern and eastern Afghanistan.

The attacks coincided with the arrival of top US military officer General Martin Dempsey in Afghanistan on an unannounced visit.

They also raise questions about the Taliban's strength ahead of the coalition's planned withdrawal next year.

US-led coalition forces are winding down their operations ahead of a scheduled full withdrawal of 100,000 remaining troops in 2014.

Taliban insurgents claimed credit on their website for the deadly Zabul strike, adding it happened near a newly constructed hospital.

In another show of strength, Taliban gunmen killed 46 people at an Afghan court complex in western Afghanistan on Thursday in a bid to free insurgents standing trial.

That attack in Farah, a province that borders Iran, was the deadliest for more than a year in Afghanistan.

ABC/AFP