"It's not too late. They can and should and must reopen real peace negotiations," Jan Egeland told AFP.
But he added, "To the best of my knowledge there is very little movement."
The Liberation Tigers of Tamil Eelam have been fighting for an independent homeland in a 35-year ethnic conflict that has claimed more than 60,000 lives.
A 2002 truce now exists only on paper as worsening fighting since late 2005 has killed more than 5,000 people, according to government figures.
"What can be done is that the two sides say, 'We want to sit down. We have now amply proven that there's no military solution to this conflict.
We spent a generation to prove that.
Now it's time to look to the future and agree on peaceful co-existence'.... That's the only way," said Egeland.
As Norway's deputy foreign minister in the early 1990s, he began work on peace efforts in Sri Lanka.
Norway remains the island's peace broker, making a bid as recently as July to revive moribund negotiations aimed