Norwegian peace mediators say they are deeply concerned by Sri Lanka's decision to withdraw from a ceasefire agreement with Tamil Tiger rebels.
On Wednesday, Colombo said it was formally withdrawing from the accord because escalating violence had made the Norwegian-brokered truce redundant.
The accord came into effect in February 2002 ahead of inconclusive peace talks.
Since mid-2006 troops and rebels have fought regular clashes, despite the deal still technically being in place.
There was no immediate response from the Tigers, who want independence for Tamils in Sri Lanka's north and east.
Meanwhile, police and security forces have been put on alert across Sri Lanka.
Norway's Environment and International Development Minister Erik Solheim, who brokered the accord, said ending it was a "serious step".
"This comes on top of the increasingly frequent and brutal acts of violence perpetrated by both parties, and I am deeply concerned that the violence and hostilities will now escalate even further," Mr Solheim was quoted as saying on the Norwegian foreign ministry website.
"The Cease Fire Agreement [CFA] had been violated nearly 10,000 times by the Tamil Tigers, so we should annul this agreement," the cabinet's spokesman on security matters, Keheliya Rambukwella, told the BBC on Wednesday.
Sri Lanka is returning to a state of war
Mr Rambukwella said "national security was threatened at every corner" and the government had taken the decision "to uphold the national security".
He said that withdrawal from the CFA was not a declaration of war against the Tamil Tigers, and would be done in a way that does not breach the conditions of the agreement.
Under the February 2002 ceasefire brokered by Norway, both the Sri Lankan government and the Tamil Tigers have the option to withdraw from the CFA after giving two weeks' written notice to the Norwegian foreign minister.
Last week, Sri Lankan Defence Secretary Gotabhaya Rajapaksa said in public that the truce between the two sides was "moribund" and that the CFA was a "joke".
Correspondents say that even though both sides acknowledged this to be true, neither wanted to be seen as the first formally to withdraw from the ceasefire.
Mr Rambukwella argued that the government had already taken steps to address the grievances of the minority Tamil community and was holding talks with moderate Tamil parties.
So far there has been no word from the Tamil Tigers on the government's announcement, but some analysts have warned that the development will only serve to polarise the two sides.
Meanwhile, security has been tightened across the island.
"We have raised an alert, especially in Colombo - we have deployed more men," news agency Associated Press quoted a police officer as saying.
About 5,000 people have been killed since early 2006, taking the total number of dead since the war erupted in 1983 to around 70,000.