An estimated 60,000 people have been killed since the Tamil insurgency began around three decades ago. The bodies of 11 men and four women, wearing Action Against Hunger (ACF) T-shirts, were found face-down in their office on Sunday in the town of Muttur, close to heavy fighting between Tamil rebels and government forces.
The rebels have accused the security forces of killing the aid workers, who were all members of Sri Lanka's minority ethnic Tamil community.
Benoit Miribel, the director general of Action Against Hunger (ACF), said his organisation was stunned by the killings which were unprecedented in its 25-year history.
"We are trying to send a team to find out what is going on in this area," Miribel told AFP in Paris. "But soldiers have prevented us from entering the town which remains completely sealed off.
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ACF is one of the hundreds of aid agencies that set up operations in Sri Lanka after an Indian Ocean tsunami wiped out much the island's coastal infrastructure and killed an est