"According to information, he was arrested in another country," Abhisit said without elaborating.
Pathmanathan, better known as KP during his decades allegedly running the LTTE's arms and smuggling networks, was the first LTTE official to acknowledge the death of Tigers founder and leader Vellupillai Prabhakaran.
Prabhakaran, along with most of the Tigers' senior leaders, was killed in the closing days of Sri Lanka's offensive against the separatists in the country's northeast.
Pathmanathan is believed to have earned millions of dollars procuring weapons for the Tigers and running smuggling operations from bases across the region including Thailand, Cambodia and Myanmar.
As one of the most senior LTTE members remaining, he declared himself the secretary general of a revamped outfit which would try non-violent means to achieve its goal of a separate state for Sri Lanka's minority Tamils.
The announcement of Pathmanathan's capture comes as campaigning for Sri Lanka's first post-war election ended on Thursday.
Voting for local councillors is to take place on Saturday in Uva province and the towns of Jaffna and Vavuniya, on the outskirts of the area formerly controlled by the Tigers.
|Sri Lanka's president has vowed to resettle 80 per cent of the refugees by year's end [EPA]
The government says campaigning has gone on without violence, unlike polls last year in the Eastern Province after the military drove the Tigers out of the area.
The government's claims cannot be independently verified as foreign media has been banned from the area.
Media rights group Reporters Without Borders said it was "unacceptable that the government should impose such a ban on nothing more than the vaguest security grounds".
The elections in the country's north are the first in the towns since 1998 and part of a pledge by Mahinda Rajapaksa, the president, to devolve power to the Tamil-majority area.
Rajapaksa has also promised to let civilians who lived in the northern areas the LTTE ran as a de facto state for many years, vote in a future province-wide election.
But around 250,000 Tamils displaced by the war are still being held in military-guarded so-called refugee camps, despite Rajapaksa's promise to resettle 80 per cent of them by the end of the year.
The government has maintained it will do so despite the need to clear tens of thousands of landmines planted across the country's north.
On Wednesday, the government said it resettled its third batch of people, among them 3,000 who fled the final part of the offensive that ended with the military declaring victory over the Tigers on May 18.
The information department said another 1,000 people were resettled in eastern districts on Wednesday.