Up to 18 family members of the suspected mastermind behind Sri Lanka’s deadly Easter Sunday suicide bombings are missing and feared dead, according to the suspect's sister.

She told CNN some of her relatives had died since the bombings and subsequent shootout with police last Friday.

Police sources told Reuters that the suspect's father and two brothers were killed when security forces stormed their safe house last week.

The bombings, which targeted churches and hotels across the island, killed at least 250 people, while leaving at least 500 injured.