Norwegian mediators say they are not optimistic about the future of Sri Lanka's peace process.
The Tigers accuse Colonel Karuna of siding with the army
After meeting Tamil Tiger leaders on Monday, Norwegian Deputy Foreign Minister Vidar Mr Helgesen said he had failed to achieve a breakthrough.
Talks between the rebels and the government stalled over a year ago.
Mr Helgesen's remarks come after a wave of violence, including a suicide bombing in the capital and almost daily killings in the island's east.
'Act of terror'
On Sunday, eight people were shot dead in a suburb of Colombo.
The Tigers said they were top aides of a rebel defector, whom they accuse of working with the army - a charge the military denies.
Mr Helgesen said the situation was disturbing and the international community looked on it with gravity.
He will now meet the government in an effort to defuse the increasingly volatile situation on the island.
A government statement on Monday condemned the killings as an "undisguised act of terror".