Five people including a doctor were killed when an ambulance was caught up in a mine attack carried out by government forces, Tamil Tiger rebels said.
The blast hit the vehicle in an area held by the rebel Liberation Tigers of Tamil Eelam (LTTE), the guerrillas said in a statement.
"An ambulance returning to its base at Nedunkerni hospital ... was attacked by a government of Sri Lanka deep penetration unit," the statement said. "Five people including all three health professionals in the vehicle were killed."
LTTE spokesman S. Puleedevan said a second attack took place around noon yesterday a few kilometers away from where the ambulance was hit, but there were no casualties.
"A doctor and three others were travelling in a white pickup truck which came under a Claymore mine attack, but fortunately no one was hurt," Puleedevan said by telephone from his base in the rebel-held town of Kilinochchi.
The Tigers said the government had carried out the attacks after the guerrillas lifted a 19-day water blockade that had deprived supplies to some 15,000 families in the island's northeast.
"Natural decency demands reciprocity from the government of Sri Lanka and not blocking humanitarian assistance to the displaced and not carrying out a Claymore attack on the health providers," the Tiger statement said.
"Is the government expressing anger at the LTTEs action of opening the sluice gate," the Tigers asked referring to earlier government claims that the military would forcibly lift the blockade.
Military officials said they were not aware of the blast inside rebel-held territory.
The blast comes a day after a car bomb attack in the capital Colombo targeting a government Tamil politician who escaped with injuries. Three others people were killed.
A Claymore mine attack in the island's central district of Kandy killed a senior police commando officer and his driver on Monday.
Water flowed to thousands of farmers in east Sri Lanka for the first time in three weeks yesterday after Tamil Tiger rebels lifted a blockade.
The army fired rockets towards Tamil Tiger positions before dawn, 15 days after jets first dropped bombs on rebel areas in a bid to wrest control a disputed sluice. The rebels said air force jets again bombed their territory.
"Water is flowing. The water level has risen 2m high. That indicates there won't be a problem of water for those villagers and farmers," said Major Upali Rajapakse, senior coordinator at the media center for national security.
"The humanitarian operation conducted by the security forces has achieved success," he said. "The fighting is on and off ... We are still consolidating the area."
The island's defense secretary said it was too early to say if the military campaign was nearly over.
The government says the Tigers must still vacate the area. The Tigers had vowed to reopen the waterway on Sunday, but the government replied with renewed artillery fire.
"Not only should the water flow, the control of the water should be [in government hands], because the water has to be managed," said Palitha Kohona, head of the government peace secretariat.
The Tigers said they had no intention of leaving the area, which Nordic truce monitors say is ill-defined, but was under de facto rebel control.
"They have no right to say this is their territory," said S. Puleedevan, head of the Tigers' peace secretariat. "We have clearly mentioned that this is tantamount to a declaration of war if the Sri Lankan armed forces launch official attacks."