An army statement blamed the Liberation Tigers of Tamil Eelam (LTTE) for the soldier's death late Wednesday night at Muhamalai on the Jaffna peninsula.
The Tigers said two rebel positions in the area had twice come under attack by government forces late Wednesday and early Thursday.
They said LTTE fighters retaliated, forcing the army to withdraw.
The Tiger statement did not make any mention of casualties.
Fighting was also reported later Thursday in the northeastern port district of Trincomalee, residents and police said.
Security forces were firing at suspected Tamil rebel positions from which they came under attack overnight, police said.
Residents reported several mortar explosions within a half-hour period.
Foreign Minister Mangala Samaraweera was in Tokyo seeking international help to curb the mounting violence which has left a four-year-old ceasefire in force only on paper.
He headed to Japan ahead of a May 30 meeting of Sri Lanka's key financial backers.
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