Thousands of civilians have been displaced by the fighting
The Sri Lankan government has attacked what it calls a "vicious coalition" of aid and humanitarian agencies for their actions over the country's civil war.
The defence ministry said those "pretending to be humanitarian and aid agencies" were prolonging the conflict "to secure their income".
Rights and aid groups have continued to criticise both the government and Tamil Tiger rebels over civilian casualties.
A new Human Rights Watch report again condemns "indiscriminate shelling".
The defence ministry website said the "vicious coalition" that had "been pretending to be humanitarian agencies, aid agencies, free media, civil rights movements, etc, have made the continued bloodshed on Sri Lankan soil a lucrative business for them".
It said the goal was "to ensure that the [Liberation Tigers of Tamil Eelam's] war would never end at any cost".
The ministry accused the Care International group of being part of the "abominable conspiracy".
Care last week reported that a local worker was killed by shellfire in a government-designated "no-fire" zone.
The ministry said "very reliable sources" indicated the man was "a hardcore LTTE cadre".
The army has restricted rebels to a small area of jungle in the north-east
Care country director Nick Osborne told Associated Press news agency the issue was "very sensitive".
"Our response at this moment is to respect the loss of a staff member and give our thoughts to his family," Mr Osborne said.
The ministry's attack came as Human Rights Watch said there were reports "of civilians being killed and wounded daily in the no-fire zone, while the Sri Lankan government continues to deny the attacks".
Brad Adams, the group's Asia director, also said: "The Tamil Tigers' use of civilians as human shields adds to the bloodshed."
The UN's human rights chief Navanethem Pillay - currently in Delhi - has appealed to the Sri Lankan government to allow independent human rights monitors to assess the situation in the north and provide better protection for civilians.
"So far I have not got any response [from the government], she said, "but I am going to carry on pressing for it."
The influential chairman of the US Senate Foreign Relations Committee John Kerry has also now written to President Mahinda Rajapaksa, saying he had "grave concerns" at reports of army shellfire in civilian areas.
The pro-rebel website TamilNet has consistently accused the army of shelling civilians.
It said another 96 civilians were killed in the designated safety zone on Monday.
No independent journalists can travel to the war zone so information provided by either side cannot be verified.
The recent Sri Lankan military offensive has restricted the rebels to a small section of jungle in the north-east.