The attack on Tuesday followed a string of guerrilla ambushes on the military and the killing of a pro-separatist member of parliament at a Christmas mass that are straining a 2002 truce to breaking point.
Prasad Samarasinghe, a military spokesman, said, "It was a claymore attack," referring to the claymore fragmentation mine used in the assault near the northern town of Point Pedro.
"Definitely the LTTE is behind this attack," he added, using the initials of the Liberation Tigers of Tamil Eelam. "No one else is capable of doing this kind of claymore mine attack in Jaffna except the LTTE."
A military official said four other soldiers were admitted to hospital after the attack, some in critical condition.
On 13 Friday, sailors were killed in an ambush by suspected Tiger rebels in the island's northwest, using claymore mines and rocket-propelled grenades.
That prompted major aid donors, Japan, the European Union and Norway to send a delegation for emergency talks with the Tigers.
Kumaratunga (C) has ruled out
the idea of a Tamil homeland
Two days later, Joseph Pararajasingam, a member of parliament for the Tamil National Alliance - the separatists' proxies in parliament - was killed at a Christmas mass in the restive eastern district of Batticaloa.
Mats Lundstrom, spokesman for the Nordic Sri Lanka Monitoring Mission overseeing the truce, said: "We are very concerned about the situation and urge both parties to show restraint.
"We are concerned about the future of the ceasefire agreement."
Sri Lanka's stock market closed nearly 7% lower, as news of the latest attack compounded earlier losses prompted by the Christmas killing.
Ceasefire monitors have stopped patrols in the northern Jaffna peninsula because of the deteriorating security situation.
The LTTE wants a Tamil homeland
carved out of the north and east
The Tigers threatened in November to resume their armed struggle to carve out a homeland for ethnic Tamils in the north and east unless they were given wide political powers in about 15% of the country where they run a de facto state.
Mahinda Rajapakse, the Sri Lankan president, who is allied to Marxists and Buddhists who refuse any concessions for the rebels, has ruled out the idea of a Tamil homeland.
Rajapakse headed to India on Tuesday on his first state visit since winning the presidency in November.
He intends to seek more Indian involvement in Sri Lanka's stalled peace process, but officials and analysts in India said he was unlikely to have much success.