An emergency meeting between the main foreign backers of Sri Lanka's peace process and Tamil Tiger rebels has ended with no breakthrough.
About 30 military personnel have been killed this month
The meeting follows the worst upsurge in violence since a ceasefire in 2002.
The envoys expressed concern about the attacks and urged new peace talks, according to a pro-Tiger website.
But there was no agreement on a venue for any talks, and the rebel leaders insisted they had no control over the violence, the website said.
On Friday, the government accused the Tamil Tigers of ambushing a road convoy, killing 13 sailors, but the rebels said they were not connected to the incident.
There was more violence on Saturday with five people, including one soldier, killed in clashes between suspected rebels and the military in the northern Jaffna region.
Saturday's emergency talks involved envoys from Japan, Norway, the EU and the US. They met the Tamil Tigers' political head, SP Thamilselvan, in the northern rebel stronghold of Kilinochchi.
The pro-Tiger website which reported the meeting said the diplomats had urged the rebels to start talks on the faltering truce.
It said the diplomats expressed concern "over the escalating violence and the necessity to start talks on effective implementation of the ceasefire agreement".
But Mr Thamilselvan is said to have told the delegation that the rebels had no control over the violence, as well as refusing to compromise on the venue for peace talks.
The government said it was prepared to sit down with the rebels anywhere in Asia.
Japan had offered to play host but the rebels rejected the government's offer, insisting talks be held in Europe.
Analysts question both side's willingness to begin negotiations if neither can agree on where they should be held.
Truce in 'jeopardy'
International donors and foreign truce monitors have said the sharp upsurge in violence, following the election of Mahinda Rajapakse as Sri Lanka's president in November, has left the ceasefire under grave threat.
"The ceasefire agreement is in jeopardy, absolutely," Hagrup Haukland, the head of the Sri Lankan Monitoring Mission is quoted as saying by news agency AP.
"The situation is alarming. There is a lot of concern about what will happen."
But the BBC's Dumeetha Luthra in Colombo says there is now real concern Sri Lanka may slip back into conflict.
The Tamil Tigers have called on the new government to come up with a political settlement within the next year or face an "intensified struggle for self-determination".