Tamil Tiger rebels have opened fire on a Sri Lankan navy ship carrying an international cease-fire monitor, a military spokesman said.
Similar incidents have happened in the past
The monitor sustained minor injuries after diving for cover when the ship was attacked near the eastern port of Trincomalee, reports said.
The Tigers deny attacking the vessel but say they returned fire.
A cease-fire between the Tigers and the government, in force since February, 2002, has been under growing strain.
Peace talks between the government and the rebels have been suspended since April 2003.
The navy patrol boat was on a routine tour when the rebels shot "75 to 100 rounds of small arms fire" at it south-east of the main harbour of Trincomalee, military spokesman Daya Ratnayake said.
The ship was flying with the Sri Lanka Monitoring Mission flag indicating that monitors are on board when it came under attack, Helen Olafsdottir, spokeswoman for the European-sponsored truce monitoring mission told the Associated Press news agency.
"It's a very serious incident, we don't take this lightly," she said.
"But we can't confirm that it was the LTTE [rebels] until more investigations are conducted."
She said only about 35 rounds had been fired.
Military spokesman Daya Ratnayake told the AFP news agency that the attack was a serious breach of the truce between the government and rebels.
The BBC's Dumeetha Luthra in Colombo says similar incidents have happened in the past, but this attack is significant because the navy boat was carrying an international monitor and a large number of rounds were fired.
The head of the Tamil Tiger's political wing in Trincomalee has denied that the Tigers were involved in the incident, according to the pro-Tiger Tamilnet website.
Last month, an attack on a top female Tamil Tiger leader in Sri Lanka raised fears of a return to violence threatening a fragile three year truce.
The Norwegian-led peace monitors have expressed their concern over the rising violence.
More than 60,000 people have died since the rebels began their fight for a homeland for minority Tamils in 1983.