The latest violence triggered fresh fears that planned peace talks may be cancelled and a civil war could restart.
Two policemen died in a claymore mine ambush on Wednesday while one soldier and four civilians were killed in a blast in a crowded marketplace in the port town of Trincomalee, the army said. Another nine people died in riots that followed.
An army spokesman said of the second attack: "It was blast by an army point by the clock tower."
"We think it was LTTE. We don't think it was a grenade. With the soldier, they only found half his body," he added, referring to the Liberation Tigers of Tamil Eelam (LTTE) rebels.
More than 30 people have now died since Friday in a dramatic rise in violence in what looks to be the bloodiest few days since a 2002 ceasefire halted two decades of civil war that killed more than 64,000 people.
Peace talks between the government and the LTTE are planned for next week in Switzerland, but diplomats are increasingly doubtful the meeting will take place.
The LTTE, who have run a de facto state across a seventh of Sri Lanka since the truce, were not immediately available for comment but they have denied carrying out recent attacks. Few analysts believe them.
In Trincomalee, which has large populations both from Tamil and Muslim communities as well as Sri Lanka's Sinhalese majority, the attack was followed by angry rioting in which officials and witnesses said Tamil shops were attacked and burnt.
Police said some 50 people were injured but a curfew had restored order. Earlier, international truce monitors described the situation as out of control.
"It is under control now," Mahendra Serasinghe, an official, told Reuters from the marketplace. "They are putting out the burning buildings."
A police sergeant at the Trincomalee hospital said wounded people from both the Sinhalese and Tamil communities were arriving with blast and burn injuries.
"Things are spiralling out of control"
Western diplomat in Sri Lanka
The government says it remains confident that talks will take place as planned, but the rebels have yet to say they will attend. Some say the peace process may be slipping away.
"Oh God," said one western diplomat when told of the latest attack. "It's very bad ... there's no other way to describe it. Time is basically running out here. Things are spiralling out of control."
The rebels say they will not attend unless the government guarantees safe passage for a Sea Tiger vessel to transport eastern Tiger commanders to their main headquarters for talks ahead of Geneva. The government says it will not.
"The government has offered a ferry which is air conditioned," head of the government peace secretariat Palitha Kohona told Reuters before the second blast.
"I think there is a certain amount of gamesmanship going on here. This government was not elected to simply concede. There is no right under the ceasefire ... for the Tigers to use their own vessels."
The LTTE also accuses the army of backing a breakaway group and using it to attack the mainstream Tigers. The army denies the charge, and say they are unable to find any armed groups operating in government areas to disarm.
Analysts fear any return to conflict could see Black Tiger suicide bombers attacking the capital, Colombo, scaring investors away from the island's $20 billion economy and hurting a country hard hit by the 2004 tsunami.
On the streets of the northern Jaffna peninsula, an almost exclusively Tamil army-held area hemmed in by rebel lines, civilians who have already weathered decades of bombs and shells say they are expecting more of the same once again.
"Peace at the moment is very doubtful," said 31-year-old liquor seller Anthony Rasiah. "I have got children. Last time when the war took place they were not born. But now they are young and where do I hide them where there is war?"