Afghan attacks raise fears over Taliban strength

The death of six Americans in the deadliest day this year for foreigners in Afghanistan has highlighted fears about the Taliban's continuing strength ahead of NATO's withdrawal in 2014.

A suicide car bomber struck a NATO convoy in the southern province of Zabul Saturday, killing three American soldiers and two US civilians, including a young female diplomat travelling with Afghan officials to distribute books to students.

Another US civilian was killed in an attack in the country's east, making it the deadliest day of insurgent violence against the coalition since July 8 last year.

The strikes came as the top US military officer General Martin Dempsey arrived in the country on an unannounced visit. They raise troubling questions about the Taliban's continuing strength before the coalition's exit.

Though the Taliban have not yet announced their "spring offensive", which started last year with a barrage of bloody attacks in early May, the traditional Afghan fighting season is beginning as the cold winter recedes.

US-led coalition forces are winding down their operations before a scheduled withdrawal of the bulk of their 100,000 troops by the end of 2014, and racing to prepare Afghan forces to assume responsibility for security.

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

The coalition has been keen to play up the Afghan forces' readiness to take over, but Saturday's violence came hard on the heels of a devastating Taliban attack on a court complex in the west of the country on Wednesday.

Gunmen went from room to room, slaughtering defenceless civilians and killing a total of 46 people -- the deadliest attack in the country for more than a year.

Last August insurgent rockets hit General Dempsey's plane as it was parked at the Bagram air field, wounding two maintenance crew. He flew out of the country unharmed using another plane.

US Secretary of State John Kerry, on a visit to Istanbul, paid tribute to 25-year-old Anne Smedinghoff, the diplomat killed in Zabul, describing her as - "vivacious, smart, capable".

"The folks who want to kill people, and that's all they want to do, are scared of knowledge. And they want to shut the doors and they don't want people to make their choices about the future," Kerry said.

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

Sharifullah Naseri, spokesman for Zabul governor Mohammad Ashraf Naseri, said he had planned to open a US-funded school in the provincial capital Qalat with the American officials and hand out books to students.

"Along the road there were some explosion, the first according to our information was a suicide car bomb followed by a second one likely by a suicide bomber on foot," the spokesman said.