The US-based rights group Human Rights Watch also condemned the forced transfers.
Brad Adams, the group's Asia director said: "The Sri Lankan government says it will never force civilians to return home after they have been displaced by fighting ... But there is clear and incontrovertible evidence that forced returns have begun."
More than 750 individuals had been sent north to Trincomalee district under a plan to return some 2,800 internally displaced persons, Human Rights Watch said.
It also accused the military of threatening to withdraw security for people who refused to return.
"The government must only return people with their free consent, and provide aid and security for all those who decide to stay until conditions improve."
Unsafe areas
The United Nations says more than 40,000 of 125,000 displaced persons in Batticaloa have fled fighting in the past week.
Thousands of people fled Trincomalee in August after fighting erupted between government forces and the Tamil Tigers.
Human Rights Watch reported that civilians it had interviewed in the area said they were afraid to go home because of the security situation, fearing fresh fighting and expressing concern over reprisal killings.
Tiger rebels have been waging an armed campaign for independence from Sri Lanka for more than three decades.
Fresh fighting
Fighting continued on Friday as four soldiers were killed and 12 seriously wounded in clashes with Tamil Tiger rebels in northern Sri Lanka, the AFP news agency reported.
Security forces tried to advance towards positions of the Liberation Tigers of Tamil Eelam (LTTE) in the Vavuniya district, but were beaten back by heavy mortar bomb fire by the rebels, military sources said.
The Sri Lankan military also shot dead a suspected Tamil Tiger rebel after he tried to blow himself up at a checkpoint in the restive northern Jaffna peninsula, an official said.
Lieutenant-Colonel Upali Rajapakse of the Media Centre for National Security in Colombo said: "A soldier shot and killed a suicide cadre who tried to embrace another soldier at a checkpoint in Jaffna. The body and the suicide jacket are there."