Sri Lanka's government has blamed Tamil Tiger rebels for weekend attacks in which at least nine soldiers died.
A landmine blast in northern Jaffna peninsula which killed six troops was a "terrorist attack" in breach of a truce accord, a government statement said.
The army also says the rebels have killed three other soldiers in separate attacks in the north since Friday.
Ceasefire monitors have said rising hostilities in the north and east could cause "irreparable damage" to security.
The rebels have not commented on the violence, which comes amid rising tensions following last month's election of hardliner Mahinda Rajapakse as president.
In the eastern port of Trincomalee, troops in full combat gear are on patrol after at least three people died in clashes between Muslims and Tamils.
Sunday's explosion in Jaffna killed more troops than any other single mine blast since the government and the rebels agreed the February 2002 truce.
The army denies any links with the breakaway rebel faction
The defence ministry said the soldiers had been travelling on a tractor trailer when it hit the mine at Kondavil near the town of Jaffna, about 300km (185 miles) north of the capital, Colombo.
One soldier was also seriously injured.
In its statement, the government warned that all necessary steps would be taken to prevent similar killings in future.
"Such provocative acts... demonstrate a lack of sincerity towards negotiations and a political settlement," it added.
Military spokesman Brig Nalin Witharanage said the blast may have been in retaliation for the shooting dead of two Tamil youths by unidentified gunmen last Thursday.
The rebels accuse the military of backing a breakaway rebel faction, which the military denies.
The Sri Lanka Monitoring Mission said the latest violence was part of a "dangerous trend of violence" in the north and the east in the last few days.
"We've had eight incidents in Jaffna this weekend between bomb and hand grenade attacks," said SLMM spokeswoman Helen Olafsdottir.
"This is the worst we've ever seen it and we are worried this could escalate out of control, causing irreparable damage to the ceasefire."
A statement from monitoring mission head Hagrup Haukland noted "countless attacks" in Tamil-majority areas.
Mr Haukland appealed to the government, the rebels and community leaders to calm the situation before it escalated further.
Sri Lanka's new president, Mahinda Rajapakse, has ruled out a separate homeland for minority Tamils in the north and east. He wants to renegotiate the truce, which ended more than 20 years of civil war in which more than 60,000 people died.
The Tamil Tigers are demanding a political settlement next year - or they say they will step up their struggle.