Six NATO personnel including a female US diplomat were killed on Saturday in two separate attacks in Afghanistan, officials said, in the deadliest day for foreign forces this year.

In the southern province of Zabul, a Taliban car bomb struck a NATO convoy killing three troops and two civilian workers, the International Security Assistance Force (ISAF) said.

According to a US security source the three soldiers were American and one of the civilian workers was from the US.

An ISAF spokesman in Kabul confirmed the explosion was a car bomb targeting a military patrol, adding that there were also Afghan civilian casualties.

In addition, an American citizen died in "an insurgent attack" in eastern Afghanistan, a statement from US-forces Afghanistan said without giving any details.

US Secretary of State John Kerry said that one of the dead in the Zabul attack was a female diplomat whom he had met during a visit to Kabul last week.

Kerry said that the American officials and their Afghan colleagues had been travelling to donate books to students at a school in Qalat, the capital of Zabul province, when they were killed in the "despicable attack".

Paying tribute to the unnamed diplomat, he described her as "smart, capable, eager to serve and deeply committed to our country and the difference she was making for the Afghan people".

The Zabul attack left four State Department staffers injured, one critically, according to Kerry.

Ashraf Naseri, the provincial governor of Zabul, told AFP he was on his way to attend an official function in Qalat when he heard the blast.

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

The strikes came as the top US military officer General Martin Dempsey arrived in the country on an unannounced visit and raise troubling questions about the Taliban's strength ahead of the coalition's withdrawal in 2014.

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

"An explosive-packed car went off this morning in Qalat city as a NATO convoy was passing. We heard it has caused heavy casualties," another provincial official told AFP on condition of anonymity.

Saturday is the deadliest day for the coalition since July 8th, 2012, when two separate attacks killed seven soldiers, also in southern and eastern Afghanistan.

Taliban insurgents, who have been waging a tough insurgency since the toppling of their regime in 2001, claimed credit for the deadly strike on their website, adding it happened near a newly constructed hospital.

The insurgents, who usually exaggerate the number of victims they kill, said: "Thirteen invaders were killed and nine others wounded whereas one guard of the governor was also killed with two others suffering injuries."

The attack, which coincides with the start of the fighting season for the Taliban, comes as US-led coalition forces are winding down their operations ahead of a scheduled full withdrawal of 100,000 remaining troops in 2014.

General Dempsey, chairman of the Joint Chiefs of Staff, arrived in the country on Saturday to assess the level of training the US will need to provide Afghan forces following the withdrawal.

Last August, insurgents' rockets hit General Dempsey's plane as it was parked at the Bagram air field and wounded two maintenance crew, according to officials. Dempsey flew out of the country unharmed using another plane.

In another show of strength, Taliban gunmen Thursday killed 46 people at an Afghan court complex in Western Afghanistan 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.