The Tamils Rehabilitation Organisation (TRO) said on Tuesday that five of its staff were grabbed near an army checkpoint in the east; but the army says the story is a fabrication.
The TRO reported on Wednesday that another five staff members and their vehicle were missing.
Hagrup Haukland, head of the Nordic mission monitoring a 2002 truce, strained to its limits by recent violence, said: "It is a very bad and sad sign.
"If it is so, then the Geneva meeting may be in danger or jeopardy."
A string of attacks on the military in the minority Tamil-dominated north and east tested a 2002 truce almost to destruction, but monitors said tensions fell after the two sides agreed last week to hold direct negotiations in Switzerland.
The TRO - headquartered in the rebel capital and seen by many international aid workers as a Tiger front - said on Wednesday five of its employees had failed to turn up for a meeting in the rebel capital of Kilinochchi that was scheduled for earlier in the week.
The report comes after the Tigers said paramilitaries had kidnapped five other TRO staff near an army checkpoint and warned that the abductions could make it difficult for them to
attend peace talks with the government this month.
Brigadier Prasad Samarasinghe, the army spokesman, said "it is a baseless allegation against the security forces".
Hagrup Haukland: The Geneva
meeting may be in jeopardy
He added: "It never happened close to our checkpoint. If it had, a lot of civilians would have seen it."
A string of telephone calls and text messages from the TRO asking for help tracking down their missing staff convinced some diplomats of the veracity of the reports.
But others say the reported abductions may have been concocted to give the Tigers a better bargaining position or maybe even to pull out of the talks - an argument backed by the Sri Lankan military.
"It would be a risky game," said one European diplomat. "If this is not true, it would eventually come out and that would really hurt them (the Tigers). But it's hard to say who did it - there are so many parties."
The TRO says both groups of five staff disappeared in areas widely said to be strongholds of the Karuna group, a breakaway Tiger faction the rebels say now acts as government-backed paramilitaries - but which this week said in a largely unverifiable statement it was now observing its own truce.
Karuna was widely blamed for an attack on the Tigers hours after the two sides agreed to talk, but diplomats say the government may have reined them in under international pressure.
But rogue army commanders and majority Sinhalese nationalists are also seen possibly wanting Geneva talks dead before they start.
"I fear the Tigers may try to delay until this is resolved. If these talks fail, it would be very difficult indeed to avoid a war"
Unnamed European diplomat
"I fear the Tigers may try to delay until this is resolved," the diplomat said. "If these talks fail, it would be very difficult indeed to avoid a war."
Suspected rebel attacks on troops in December and January tested the ceasefire so much that monitors asked if it still held at all. Tensions only reduced last Wednesday after Erik Solheim, the Norwegian envoy, brokered an agreement for the two sides to meet.