Relief officials said the protests were in response to the reported abductions of staff from an aid group closely tied to the Liberation Tigers of Tamil Eelam (LTTE), who blame the government for the incident and say it will delay talks scheduled in Geneva for mid-February.

"It seems they're not letting people cross into Tiger-controlled areas," said Helen Olafsdottir, spokeswoman for the Nordic-staffed mission monitoring a 2002 ceasefire. "They're burning tyres at the crossing points."

The barricades appeared to be limited to the east, she said. Sri Lanka had seemed to be stepping back from the brink of civil war in late January when the two sides agreed to meet in Switzerland and talk about stopping violence that had killed more than 200 people in two months.

But after the reported abduction of 10 workers from the Tamils Rehabilitation Organisation (TRO), effectively the relief arm of the de facto Tamil state, the rebels said they would not accept the government's suggested dates of Feb. 15 and 16, and were aiming for talks at the end of the month instead.

If the talks fail or are cancelled, diplomats say, it would be difficult to avoid going back to a two-decade civil war that has already killed more than 64,000 people. Norway said Solheim was due to meet chief rebel negotiator Anton Balasingham later on Monday in London to try to resolve the new impasse.