Mr Pathmanathan has called on the rebels to silence their guns
The new leader of the Tamil Tiger rebels, Selvarasa Pathmanathan, has been arrested, officials say.
He was arrested in a a south-east Asian country but he is currently being held in Sri Lanka.
The rebels have confirmed the arrest. Mr Pathmanathan became the leader of the remnants of the Tigers after their defeat in May by Sri Lankan forces.
But the BBC's Charles Haviland in Colombo says the exact circumstances surrounding his arrest remain unclear.
Mr Pathmanathan is now in the custody of the Sri Lankan authorities after reportedly being picked up earlier in a south-east Asian country.
Initial reports from Sri Lankan military officials suggested the arrest took place in Thailand, but Bangkok later denied such reports.
Thai government spokesman Panitan Wattanayagorn denied Mr Pathmanathan was arrested in his country but conceded there were "reports that he has been travelling in and out of Thailand".
A pro-rebel website said Mr Pathmanathan was abducted from the Malaysian capital, Kuala Lumpur and blamed Sri Lankan and Malaysian intelligence for his disappearance.
Malaysian Prime Minister Najib Razak told reporters he could neither confirm nor deny the report.
The full details about the circumstances of the arrest are not likely to emerge soon, our correspondent says.
Speaking to journalists in Colombo, defence spokesman Keheliya Rambukwella, would only say that Mr Pathmanathan was arrested "in the Asian region" within the past couple of days.
He described the arrest as "very very significant" because, he said, while Mr Pathmanathan was still at large many people thought the Tamil Tigers were still "alive and kicking".
Mr Pathmanathan is believed to have run the LTTE's arms procurement and smuggling networks and was wanted by Interpol.
He is also wanted in India in connection with the assassination of former Indian Prime Minister Rajiv Gandhi by a suspected Tamil female suicide bomber in 1991.
The war in Sri Lanka ended earlier this year
"He is... being questioned by investigators at the moment," Sri Lankan military spokesperson Brig Udaya Nanayakkara told the BBC's Ethirajan Anbarasan.
At the close of Sri Lanka's war he issued a statement saying the Tigers would try to achieve their goals using non-violent methods. Only two weeks ago he was named as their new head.
As the new rebel leader, Mr Pathmanathan said the LTTE would try non-violent methods to achieve their goal of a separate state for the Tamil minority.