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

Eleven Bangladeshi nationals, who used to work at a copper factory owned by Inshaf Ibrahim involved in the Shangri-La hotel bombing in Colombo on April 21, have been sent back by Sri Lanka.

Mohamed Hashim Mohamed Zahran was 12 years old when he began his studies at the Jamiathul Falah Arabic College. He was a nobody, with no claim to scholarship other than ambition.

The bodies of 15 people, including six children, are discovered at the site of a fierce overnight gun battle on the east coast of Sri Lanka, a military spokesman said on Saturday, six days after suicide bombers killed more than 250 people.

Sri Lankan President Maithripala Sirisena says police are looking for 140 people believed to have links with the Islamic State group over the Easter Sunday attacks on churches and hotels that killed at least 253 people.

What does it mean for the people when a country is put in fear and terror, a country that already has a whole history of violent experiences behind it?

Sri Lankan officials revise the death toll from Easter Sunday bombings down by about 100 on Thursday, blaming the difficulty in identifying body parts at bomb scenes for the earlier inaccurate number.

Prime Minister Sheikh Hasina in parliament urges people to remain alert and inform law enforcement agencies instantly about any suspicious terror and militant activities in their respective areas.

The scale and sophistication of the Easter Sunday attacks on churches and hotels in Sri Lanka suggested the involvement of an external group such as Islamic State, the US ambassador says as the death toll jumped to 359.

Standing under a black flag of the Islamic State, eight men declare their loyalty to the group's leader, Abu Bakr Al-Baghdadi. "We pledge allegiance ... and to obey him on everything either in easy or difficult conditions," they say, before praising Allah.