COLOMBO (AFP) - Tamil Tiger rebels Saturday sank a merchant vessel used by Sri Lanka's navy while it was moored at the eastern coastal town of Trincomalee, hours before the government held elections in the area.

The guerrillas said their Sea Tigers underwater naval commandos launched a pre-dawn attack on the boat while it was being loaded with explosives headed for troops operating in the island's north.

"Commandos from Kangkai Amaran unit of the Sea Tigers took part in the naval mission in destroying the 80-metre long vessel," the Liberation Tigers of Tamil Eelam (LTTE) said.

The Sri Lanka navy said the "MV Invincible" A-520 logistic vessel sank when an underwater explosion ripped through it, while it was moored at the Ashroff jetty in Trincomalee harbour, some 300 kilometres (187 miles) from Colombo.

"No lives were lost due to the explosion," the defence ministry statement said.

Built in the 1970's, the A-520 was a merchant vessel handed over to the Sri Lankan Navy, following a court order after it was apprehended while transporting 254 illegal migrants off southern coast in Tangalle in 2003.

The explosion came hours after a bomb ripped through a crowded cafe in Ampara, the island's east, on Friday killing 12 people and wounding at least 36.

Minutes before the blast, a convoy carrying a key government minister had passed the area, eyewitnesses said but police said the target of the attack was unclear.

The defence ministry blamed the attack on the LTTE, saying the rebels wanted to disrupt normalcy in the area as the government prepared to hold elections in the eastern province.

Under a million residents in Batticaloa, Trincomalee and Ampara will cast their ballots on Saturday to elect 37 officials to run the eastern provincial council.

The local polls, the first in the region after nearly two decades, comes after security forces wrestled the island's east from the LTTE last July.

The government has thrown a heavy security blanket around the eastern district pouring over 20,000 security forces personnel into the area to ensure a smooth poll, the ministry said.

The election is seen as a litmus test for President Mahinda Rajapakse's hawkish government as it escalates the war against the rebels to regain vast swathes of land under guerrilla control in the north.

Rajapakse is hoping the elections will deliver a show of public support despite concerns about the human and economic costs of the latest round of fighting.

Since then, Colombo says it has been trying to win the "hearts and minds" of the east's "liberated" Tamils -- a strategy it wants to take to the rebel-held north.

Although the eastern province is described as being under full government control, LTTE cells still operate.

Tens of thousands have died since the Tamil Tiger rebels launched a separatist campaign in 1972, to carve out a homeland for minority Tamils in the island's north and east.

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