A pregnant woman and three of her relatives have been killed in a bomb explosion in the northern Afghan province of Sar-e Pol, officials say. The woman was on her way to hospital in an ambulance to give birth when the vehicle struck a landmine, they said. The Taliban regularly use roadside bombs against Afghan and NATO forces. At least 14 prisoners, including insurgents, are still reported to be at large after militants attacked a jail in Sar-e Pol on Thursday night. \"The woman was being taken to hospital to give birth,\" interior ministry spokesman Sediq Sediqqi told the AFP news agency. \"Along the road their vehicle struck a mine, planted by the Taliban, and she was killed. Three of her relatives travelling with her were also killed.\" The UN says that the number of civilians killed over the last five years has risen steadily, reaching a record of 3021 in 2011 -- with the vast majority caused by insurgents.