International peace monitors in Sri Lanka say a grenade blast at their premises was the first direct attack on their organisation.
Several vehicles were damaged but no-one was hurt in the overnight blast in the eastern town of Batticaloa, which comes amid rising violence.
Separately, two sailors were killed by a landmine in the north of the island.
The past month has been the bloodiest since a truce almost four years ago, with more than 100 people killed.
On Friday, the Norwegian-led monitors issued a statement blaming both Tamil Tiger rebels and the government for the continued violence.
Civil war fears
They have repeatedly warned that it threatens a return to civil war.
A series of attacks against the Sri Lankan military has left more than 70 dead in the past month.
The Tamil Tigers have denied involvement, but the monitors now say that rebel involvement cannot be ruled out.
Tamil Tiger supporters say more than 40 Tamils have been killed by the security forces in a series of attacks since the start of December. Others blame some of those deaths on the rebels or other armed groups.
International mediation efforts have so far failed to persuade the government and the rebels to resume peace talks, which stalled in 2003.
The Tamil Tigers want a separate homeland in the north and east of Sri Lanka.
More than 60,000 people died during two decades of conflict with Sri Lanka's security forces.
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