Sri Lanka's Tamil Tiger rebels have formally said they will not attend peace talks in Geneva because of restrictions on their movement.
There has been a recent upsurge in violence
They said security "hurdles" had to be removed so they could hold a crucial internal meeting first.
The peace talks, which had already been postponed, were due to take place on 24-25 April.
Meanwhile the army said rebels had killed four soldiers in a mine attack in the northern town of Vavuniya.
The Tamil Tiger rebels wrote a letter to Norwegian peace brokers saying: "Until the hurdles in front of us to attend Geneva talks are removed and a more conducive environment created, our team is unable to come to the talks."
The Tamil Tigers' eastern commanders were to have made the journey north for an internal meeting on Saturday.
The Tigers say they did not agree to navel vessels' presence
After days of negotiations, both the government and the rebels had agreed to a civilian vessel transporting the regional leaders, escorted by unarmed international truce monitors.
Sri Lankan navy ships were to have observed from a distance.
But the Tamil Tigers cancelled at the last minute, saying they had not agreed to the navy vessels being present.
"Ceasefire monitors had told us that the navy would not interfere with our travel in any way," S P Tamilchelvan, political wing leader of the Liberation Tigers of Tamil Eelam told the BBC News website.
But ceasefire monitors insist Tamil rebels had agreed to the Sri Lankan navy vessel escorting them.
"It was part of the agreement. The rebels should have read the clauses carefully. We are frustrated," said Helen Olafsdottir, spokesperson for the Sri Lanka Monitoring Mission.
"It looks like both parties seemed to have completely lost sight of what is important. If Geneva peace talks don't happen then we can expect an escalation of violence as it happened a few months ago," she said.
Upsurge in violence
The peace negotiations were to have been the second round of talks on strengthening the implementation of the four-year-old ceasefire.
The first round was held in February in the wake of serious violence in the preceding two months, after Norway indulged in shuttle diplomacy to bring both parties to the negotiating table.
At least 160 people - including about 100 soldiers and sailors and many civilians - died in the upsurge of violence since last December.
Tamil Tiger supporters say more than 50 Tamils have been killed by the security forces in the same period, while others blame some of those deaths on the rebels or other armed groups.
The sides were originally scheduled to meet on 19 April, but the rebels sought a postponement.