The Sri Lankan government and Tamil Tiger rebels must resume peace talks immediately to prevent a return to civil war, peace monitors have warned.
A widow (r) watches the body of her soldier husband, killed on Tuesday
Norwegian peace envoy Erik Solheim said there was "no time to lose".
His message came a day after 11 soldiers died in a mine attack in the north, bringing the number of military killed in December to more than 40.
President Mahinda Rajapakse met India's PM in Delhi on Wednesday to urge a greater Indian presence in the process.
Both nations agreed peace talks must resume at once.
The BBC's Dumeetha Luthra in Colombo says December has been the bloodiest month since the Sri Lankan truce came into effect in 2002. Peace talks have been stalled since April 2003.
The government has accused the rebels of carrying out the attacks against military forces, but the Tigers deny involvement.
Mr Solheim, Norway's international development minister, said Oslo was "deeply concerned" for the peace process.
"The high level of violence and the tragic loss of life are putting the ceasefire agreement at risk and will make it very difficult to secure further progress in the peace process," he said.
The US also called for a renewed effort in the peace process.
State Department deputy spokesman Adam Ereli said: "We call on both sides to embrace peace and work together to build a future for Sri Lanka that is prosperous and secure."
Last month Tiger leader Velupillai Prabhakaran issued an ultimatum to the country's new government to come up with a political settlement within the next year or face an "intensified struggle for self-determination".
President Mahinda Rajapakse then made an offer to hold talks with the Tigers anywhere in Asia, but the rebels insisted a meeting be held in Europe.
Mr Rajapakse held talks with Indian Prime Minister Manmohan Singh in Delhi on Wednesday on his first visit abroad since taking office.
He has said he wants greater Indian involvement in the peace process.
Indian foreign ministry spokesman Navtej Sarna said after the talks that Delhi was deeply concerned at the recent upsurge of violence.
However, he would not be drawn on a greater Indian involvement in the peace process.
In Tuesday's attack at Puloly, 25km (15 miles) north-east of Jaffna town, 11 soldiers died when the vehicle carrying them hit a landmine.
Last week, 13 navy sailors were killed in a similar attack in Mannar district.
On Sunday, Joseph Pararajasingam, a Tamil MP representing a party with close links with the Tamil Tigers, was shot dead in the eastern city of Batticaloa while celebrating a Christmas Mass.
The Tigers want a separate homeland in the north and east. The two-decade conflict prior to the 2002 ceasefire claimed about 64,000 lives.