There has been no comment on the attacks from the Tamil Tigers who have been in a conflict with government forces for more than two decades.
The military has stepped up its push to defeat the group from territory they hold in the north.
The bus bombings on Friday were the latest in a series of attacks on civilian transport across the country.
On Wednesday, 18 people were injured when suspected LTTE fighters set off a bomb alongside a packed commuter train, and on May 26
another attack on a train left nine peopledead and 84 others wounded.
"This could be the start of a worsening cycle of targeting civilians," Jehan Perera, an activist for the independent National Peace Council, said.
"The government must also be careful with its own operations."
He said that the bus bombings were likely to be a "tit-for-tat kind of retaliation" by the Tamil Tigers who say that the military has killed civilians in the northern areas they control with mines and air raids.
Mahinda Rajapakse, Sri Lanka's president, has accused the Tamil Tigers of trying to provoke the island's ethnic Sinhalese majority.
"This brutality ... shows the efforts of the LTTE to provoke a backlash against the Tamil people from which it hopes to gain," he
More than 70,000 people have been killed in fighting since the conflict began in 1983. A peace process broke down nearly two years ago and fighting has steadily escalated since.