The death of at least five Americans, in the deadliest day for foreigners in Afghanistan this year, has highlighted fears about Taliban 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 US soldiers and two civilians, including a female American diplomat travelling with Afghan officials to distribute books to students. The nationality of the second civilian was unclear. 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 ahead of the coalition's withdrawal in 2014. 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 Afghanistan 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 paid tribute to the diplomat killed in what he called the "despicable attack" in Zabul. 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. 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.