Catholic schools are to start reopening after holidays were extended due to suicide bombings. (File)

Sri Lanka has tightened security ahead of the reopening Tuesday of some Catholic schools for the first time since the Easter bombings that killed 258 people, the army chief said.

Lieutenant General Mahesh Senanayake said security forces would ensure that schools were not targeted by ISIS responsible for the April 21 bombings.

"Army and sister services are fully and confidently assisting law enforcement authorities to apprehend the culprits," Senanayake said Monday, adding that operations had shown "marked progress" without giving details.

Dozens of private Catholic schools are to start reopening Tuesday after Easter holidays were extended because of the suicide bombings on three churches and three hotels.

A Roman Catholic spokesman in Colombo said schools would reopen on a staggered basis over the next two weeks and that school authorities had been asked to ensure safety.

"In some places children will not attend classes in their uniforms," the spokesman said.

"Advanced Level classes will start on Tuesday but primary and middle school will reopen gradually in the weeks to come."

More than 10,000 public schools reopened earlier this month, but attendance has been low.

On Sunday, President Maithripala Sirisena marked the 10th anniversary of the end of the country's Tamil rebel war by vowing to crush ISIS blamed for the Easter bombings that also left 500 injured.

"We have been able to arrest all those who were behind the Easter attacks. Some have been killed in confrontations," he said.

"This gives me confidence that we can completely eliminate the threat from international terrorism."

Muslims account for 10 percent of the population in the mainly Buddhist nation, which has been under a state of emergency since the attacks that were also claimed by the ISIS.