A senior Norwegian envoy has said Sri Lanka is at a crossroads, as he holds talks to prevent a return to civil war.
Peace monitors say both sides are to blame for rising violence
Erik Solheim met Sri Lanka's foreign minister in Colombo as well as US Undersecretary of State Nicholas Burns.
Mr Burns warned Tamil rebels they would never have ties with the US unless they ended "reprehensible" attacks.
Mr Solheim helped broker a truce in 2002, but violence that has left more than 120 people dead in recent weeks has prompted fears it no longer holds.
As he arrived, three soldiers died in a landmine explosion in the east, which the army blamed on the Tamil Tigers.
"Everyone is worried with the present deteriorating security situation," Mr Solheim told journalists in Colombo after his meetings.
"It is hard to see the present situation continuing indefinitely. Sri Lanka is at a crossroads."
Mr Burns said the US supported Sri Lanka's territorial integrity.
"We hope the LTTE [Liberation Tigers of Tamil Eelam] understands that it will have no relations with my country, and for that matter any effective relations with any country in the world, on the barrel of the gun," he told reporters after meeting President Mahinda Rajapakse.
The BBC's Dumeetha Luthra in Colombo says Mr Solheim's visit is being seen as a crucial attempt to save the four-year-old ceasefire.
MAJOR SRI LANKA ATTACKS
12 January: Nine sailors die in mine blast near Vavuniya
7 January: 13 sailors killed off coast of Trincomalee
2 January: Five Tamil youths killed in Trincomalee
27 December: Jaffna mine kills 11 soldiers
23 December: Mine blast kills 13 sailors in Mannar district
6 December: Jaffna mine kills seven troops
4 December: Seven troops die in Jaffna mine blast
Although neither side has officially withdrawn from the peace process, expectations that the envoy will bring them to talks are not high, she says.
Mr Solheim is due to meet President Rajapakse on Tuesday before travelling north to meet Tamil Tiger rebel leader Velupillai Prabhakaran.
Top rebel negotiator Anton Balasingham is also in Sri Lanka and is expected to join talks between Mr Solheim and Mr Prabhakaran.
Both the government and the rebels have said they are willing to discuss strengthening the truce - but they have been unable to agree where the talks should be held.
The latest violent incident came in Batticaloa district. A bomb went off as troops were sweeping a main highway for mines, a military official told the AFP news agency.
At least 120 people - including about 80 soldiers and sailors and many civilians - have died in the upsurge of violence since early December.
The attacks on the military have been blamed on the rebels, who deny involvement.
Analysts say they are trying to provoke the government into retaliation and war.
So far the government has not responded, although soldiers have been accused of targeting Tamil civilians.
Tamil Tiger supporters say more than 40 Tamils have been killed by the security forces in a series of attacks since the start of December. Others blame some of those deaths on the rebels or other armed groups.
The Tamil Tigers want a separate state in the north and east of Sri Lanka.
More than 60,000 people died during two decades of conflict with Sri Lanka's security forces.