A claymore mine was set off against a moving train in eastern Sri Lanka on Wednesday, police said.
A military spokesman in Colombo said five people were injured in the explosion close to Valachchenai, 280 km east of the capital Colombo.
Two more unexploded claymore mines have been found in the vicinity.
The military in its stepped up operations against Tamil Tiger rebels had earlier claimed to have recaptured the eastern province following a series of successive operations over the last six months.
More than 4,200 people have been killed in the past 16 months of fighting in Sri Lanka.