Soldiers stand guard near the wreckage of a bus, destroyed by a bomb, in Colombo, Sri Lanka, in February. United Nations officials say more than 200 people have died in violence and attacks so far this year as the country's civil war rages on. . (Gemunu Amarasinghe/Associated Press)

Two bomb attacks on buses in Sri Lanka killed at least 22 people on Friday, security officials said, the latest in a series of assaults by suspected Tamil Tiger rebels on the country's transport system.

In the first attack Friday, 20 people were killed and 64 wounded when a roadside bomb exploded during morning rush hour near a crowded bus in the capital Colombo, the military said.

Later in the day, an explosion hit a bus in the central town of Polgolla, killing at least two and wounding 20 others, police said.

Sri Lanka's army blamed the Colombo bus attack on Tamil Tiger rebels, who are fighting for a separate homeland in the north and east of the country.

The rebel group did not immediately comment on the blast.

"Tamil Tigers detonated a roadside bomb about 7:35 a.m., targeting a passenger bus in the Colombo suburb of Moratuwa," said military spokesman Brig. Udaya Nanayakkara.

Rebel spokesman Rasiah Ilanthirayan did not respond to calls seeking comment, but the rebels routinely deny such attacks.

The Tigers, blamed for scores of suicide bombings and other attacks on civilians, are listed as a terrorist group by Canada, the United States, the European Union and India.

More than 200 have died this year: UN

The United Nations Office for the Coordination of Humanitarian Affairs says more than 200 civilians have died in bombings since the beginning of the year in both government-controlled ethnic majority Sinhalese areas and northern rebel-held territory.

The Tigers have fought since 1983 to create an independent homeland for ethnic minority Tamils, who have been marginalized by successive governments controlled by the majority Sinhalese. More than 70,000 people have been killed.

Fighting has escalated along the northern front lines since the government withdrew from a long-ignored ceasefire in January.

The government has pledged to retake territory controlled by the rebels in the north and crush them by the end of the year. But diplomats and other observers say the army is facing steep resistance.

A four-year peace process between the government and the rebels ended in 2006.

With files from the Associated Press