The father and two brothers of the suspected mastermind of Sri Lanka's Easter Sunday bombings were killed when security forces stormed their safe house two days ago, police sources and a relative of the suicide bombers told Reuters on Sunday.

Zainee Hashim, Rilwan Hashim and their father Mohamed Hashim, who were seen in a video circulating on social media calling for all-out war against non believers, were among 15 killed in a fierce gun battle with the military on the east coast on Friday, four police sources said.

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Niyaz Sharif, the brother-in-law of Zahran Hashim, the suspected ringleader of the wave of Easter Sunday bombings that killed over 250 people in churches and hotels across the island nation, told Reuters the video showed Zahran's two brothers and father.

Three of the people killed in Friday's gun battle were the same people who were seen in the undated video on social media, in which they discus martyrdom and urge their followers to kill all non believers, police sources said.

Sri Lanka has been on high alert since the attacks on Easter Sunday, with nearly 10,000 soldiers deployed across the island to carry out searches and hunt down members of two local Islamist groups believed to have carried out the attack.

Authorities have detained more than 100 people, including foreigners from Syria and Egypt since the April 21 bombings.

In the video, Rilwan Hashim is seen calling for all out 'jihad', or holy war, while children cry in the background.

"We will destroy these non-believers to protect this land and therefore we need to do jihad," Rilwan says in the video, sitting beside his brother and father.

"We need to teach a proper lesson for these non-believers who have been destroying Muslims."

Authorities suspect there may be more suicide bombers on the loose. Defence authorities have so far focused their investigations on international links to two domestic groups they believe carried out the attacks, the National Thawheedh Jamaath and Jammiyathul Millathu Ibrahim.

Islamic State has claimed responsibility for the Easter bombings, and on Sunday the group said three of its members clashed with Sri Lankan police for several hours in Friday's gun battle on the east coast before detonating their explosive vests, the militant group's news agency Amaq said.

The group said 17 policemen were killed or injured in the attack, but the Sri Lankan military has denied this. A police source told Reuters two policemen were slightly injured in the battle.

Police have said six children were among the other 12 people who died in the gun battle, but have not released further details.