The office of the pro-government Eelam People's Democratic Party (EPDP) came under attack in the town of Batticaloa, 303 kilometres (190 miles) east of here by road, a military official in the area said by telephone.
At least four men were wounded in the bombing, the military said.
The military blamed the bomb attack on the rebel Liberation Tigers of Tamil Eelam (LTTE), but there was no immediate comment from the guerrillas who were outlawed by the European Union Monday as a terrorist group.
In the island's northern district of Batticaloa, two EPDP men distributing the party newspaper were shot dead Thursday, officials said.
The Tigers have accused the EPDP of collaborating with security forces to attack them.
The violence came as US Assistant Secretary of State for South and Central Asian affairs, Richard Boucher, arrived here for talks with Sri Lankan leaders and businessmen on the island's troubled peace process.
Boucher stopped over in Colombo on his way from Tokyo where earl