Two Tamil aid workers have turned up safe in eastern Sri Lanka after being kidnapped but eight others are still missing, their employers say.
The alleged abductions come as Sri Lankans fear civil war
The Tamil Rehabilitation Organisation, which has close links with the Tamil Tigers, said two groups of its workers were kidnapped on Monday and Tuesday.
It said paramilitaries were to blame. The rebels said abductions could prevent peace talks due this month.
The government dismissed the reports. Ceasefire monitors are investigating.
"They were abducted and they have testified to that effect," TRO spokesman Arjunan Ethirveerasingam said of the two workers, who are both women.
"We're still not sure exactly what happened. They had been with their families from yesterday, but they were too scared to come out," he told Reuters news agency.
The alleged abductions come after a recent surge in violence which has raised fears of a return to all-out war in Sri Lanka.
Earlier in the day, the head of the Sri Lanka Monitoring Mission said the reported abductions were "a bad sign" ahead of talks between the government and the rebels due in Switzerland this month.
"If it is so, then the Geneva meeting may be in danger or jeopardy," Hagrup Haukland told Reuters news agency.
The TRO says both groups disappeared in Batticaloa district.
It said the first group were abducted by armed paramilitaries at Welikanda, near Batticaloa, about 150km (90 miles) east of the capital, Colombo.
The Sri Lankan government said it had no knowledge of the incident and that it would investigate.
The rebels have hinted that a breakaway faction of their group, who they allege are backed by the government, are behind a series of recent disappearances.
The rebels have warned that kidnappings could threaten the peace talks.
International donors and peace monitors have warned that a four-year-old ceasefire is close to collapsing following a sharp upsurge in violence.
At least 120 people - including about 80 soldiers and sailors and many civilians - have died in the upsurge of violence since early December.
The attacks on the military have been blamed on the rebels, who deny involvement.
Tamil Tiger supporters say more than 40 Tamils have been killed by the security forces in a series of attacks since the start of December. Others blame some of those deaths on the rebels or other armed groups.
More than 60,000 people died during two decades of conflict in Sri Lanka.