The group said 17 policemen were killed or injured in the attack. The group did not give evidence for its claim.
Meanwhile, Sri Lanka’s President Maithripala Sirisena on Saturday outlawed two Islamist groups suspected to be behind the suicide bombings on churches and hotels while the wife and child of the suspected ringleader were wounded during a military raid in safe house, his family and police said.
The National Thawheedh Jamaath (NTJ) and Jamathei Millathu Ibrahim were banned under his emergency powers, he said in a statement, nearly a week after the Easter Sunday attacks that killed more than 250 people.
Authorities could not act earlier to ban the two little known groups because the law required them to show firm evidence against them, officials said.
Police believe the suspected mastermind of the bombings, Mohamed Hashim Mohamed Zahran, led either the NTJ or a splinter group.
Less is known about Jamathei Millathu Ibrahim, whose members are also believed to have played a role in the bombings.
Nearly 10,000 soldiers have been deployed across the island to carry out searches and boost security since the bombings in three churches and four hotels, most of which were in the capital Colombo.
Security forces have detained 100 people, including foreigners from Syria and Egypt, police said.
A gun battle erupted on Friday evening during a raid on a safe house in Sainthamaruthu in Ampara district on the island’s east coast, killing at least 15 people including three people with suicide vests and six children, a military spokesman said.
The wounded included the wife and a daughter of Zahran, his family said. “Yes, the wife and daughter were injured in the attack,” said Mohamed Hashim Mathaniya, sister of Zahran.
“I was asked to come to identify them but I am not sure I can go,” she told Reuters from the town of Kattankudy in the east where Zahran was originally based.
Zahran’s driver was detained in a separate raid, according to a police statement. Bomb-making materials, dozens of gelignite sticks and thousands of ball bearings were found in a search of a separate house in the same area, along with Islamic State banners and uniforms, the military said.
Zahran appeared in a video released by IS days after the bombing, the only one showing his face while seven others were covered.
In the video the men stand under a black Islamic State flag and declare their loyalty to its leader, Abu Bakr Al-Baghdadi.
Fear of more attacks
Authorities have said there could be more attacks against religious centres. Last Sunday’s bombings shattered the relative calm that the Buddhist-majority country has seen since a 26-year civil war with mostly-Hindu ethnic Tamil separatists ended a decade ago.
Sirisena and the government of Prime Minister Ranil Wickremesinghe have faced strong criticism after it emerged that India had repeatedly given warnings of the possibility of attacks.
Both Sirisena and Wickremesinghe have said intelligence was not shared with them, exposing rifts at the top of the government and raising questions about its ability to deal with the security crisis.
Police said on Friday they were trying to track down 140 people they believe have links with IS.
The president said some of the country’s youth had been involved with the group since 2013 and that there were drug trafficking links.
Muslims were urged to pray at home on Friday after the State Intelligence Services warned of possible car bomb attacks, amid fears of retaliatory violence.
Many have fled their homes amid bomb scares, lockdowns and security sweeps. The archbishop of Colombo, Cardinal Malcolm Ranjith, told reporters he had seen an internal security document warning of further attacks on churches and said there would be no Catholic masses celebrated anywhere on the island this Sunday.
Sri Lanka’s 22 million people include minority Christians, Muslims and Hindus. Until now, Christians had largely managed to avoid the worst of the island’s conflict and communal tensions.
Most of the bombing victims were Sri Lankans. The dead also included 40 foreigners, including British, US, Australian, Turkish, Indian, Chinese, Danish, Dutch and Portuguese nationals.