Norwegian observers in Sri Lanka were fired on by government forces during attacks on Wednesday which killed at least 65 civilians and injured about 300, Norwegian Aid Minister Erik Solheim told Norway's NRK broadcaster.
"This is very serious. Government forces fired at unarmed people with the aim of killing them," Solheim, who is also the chief peace-broker for Sri Lanka, told NRK.
Norway heads the Sri Lanka Monitoring Mission (SLMM).
Government forces on Wednesday shelled a refugee camp in eastern Sri Lanka. The victims were ethnic Tamils who had been displaced by the recent upsurge in shelling in the region.
The bloodshed came as security forces and Tamil Tiger rebels accused each other of trying to spark full-scale war.
Nearly 3,300 people have been killed in fighting in the past year despite a 2002 truce in Sri Lanka's separatist conflict.
The conflict has killed more than 60,000 people since 1972, with rebels fighting for a separate homeland for the Tamil minority in the Sinhalese majority country.