COLOMBO, Nov 19 (Reuters) - Nordic truce monitors said on Sunday Sri Lankan troops opened fire on a group of agriculture students at close range in the island's north on Saturday, killing five, after a deadly rebel ambush on government forces. A military spokesman said ground troops told him the civilians were killed in crossfire after a Tamil Tiger blast in the northern district of Vavuniya killed five soldiers on Saturday, and that police were investigating. "These soldiers fired indiscriminately at a group of students who had thrown themselves on the ground seeking safety after an LTTE (Tamil Tiger) Claymore mine blast nearby," Helen Olafsdottir, spokeswoman for the Sri Lanka Monitoring Mission that oversees the 2002 ceasefire, told Reuters. "Witnesses say that soldiers jumped over the fence, into the agricultural school premises, and opened fire," she added. "They shot from close range, five of the students were killed and at least 10 others were injured." The killings are the latest in a series of abuses monitors have pinned on both sides in a new chapter in the island's civil war, which has cause more than 67,000 deaths since 1983 -- more than 3,000 of them this year alone.