Sri Lanka's president has asked for the resignations of the defence secretary and national police chief, a dramatic internal shake-up after security forces shrugged off intelligence reports warning of possible attacks before Easter bombings that killed over 350 people, the president's office said Wednesday.

It wasn't immediately clear who would be replacing them, but President Maithripala Sirisena said during a televised speech Tuesday that he planned to change the head of the defence forces within 24 hours.

The Islamic State group has claimed responsibility for the attacks, which struck Christians worshipping in three churches and people at three luxury hotels. Authorities remain unsure of its involvement, though many suspects experienced foreign militants were advising, funding or guiding the attackers.

Sri Lanka's junior defence minister has blamed breakaway members of two obscure local extremist Muslim groups and said many of the suicide bombers were highly educated and came from well-off families.

'Their thinking is that Islam can be the only religion in this country,' Ruwan Wijewardene told reporters. 'They are quite well-educated people,' he said, adding that at least one had a law degree and some may have studied in the UK and Australia.

Leaders have vowed to overhaul the country's security apparatus after acknowledging that some intelligence units were aware of possible attacks before the Easter bombings.

US Ambassador Alaina Teplitz told reporters that 'clearly there was some failure in the system,' but said the US had no prior knowledge of a threat before the attacks, the worst violence in the South Asian island nation since its civil war ended a decade ago.

Teplitz called that breakdown in communication 'incredibly tragic.'

Government statements about the attacks have been confused and sometimes contradictory, with police spokesman Ruwan Gunasekara telling reporters Wednesday that there were nine suicide bombers — two more than officials said one day earlier.

One of the additional suicide bombers was the wife of another bomber, Gunasekara said. The woman, two children and three policemen died in an explosion as authorities closed in on her late Sunday, hours after attacks were launched against three churches and three hotels. The ninth suicide bomber has not been identified, though two more suspects were killed in a later explosion on the outskirts of Colombo.

Gunasekara said 60 people have been arrested so far. A team of FBI agents and US military officials were helping in the investigation, Teplitz said.

Officials say all of the main suicide bombers were Sri Lankan.

'We are conducting investigations at the moment to see if there is any direct link to any international organizations,' Wijewardene said.

The Islamic State group's Aamaq news agency released an image it said showed the attackers' leader standing amid seven others with covered faces. It provided no other evidence for its claim.

The group, which has lost all the territory it once held in Iraq and Syria, has made a series of unsupported claims of responsibility for various attacks around the world.

Sri Lankan authorities had earlier blamed a local extremist group, National Towheed Jamaat, whose leader, alternately named Mohammed Zahran or Zahran Hashmi, became known to Muslim leaders three years ago for his incendiary online speeches. On Wednesday, Wijewardene said the attackers had broken away from National Towheed Jamaat and another group, which he identified only as 'JMI.'

Teplitz declined to discuss whether US officials knew about National Towheed Jamaat or its leader before the attack. 'If we had heard something, we would have tried to do something about this,' Teplitz said.

The country has been on heightened alert since the attacks, with police setting off a series of controlled explosions of suspicious objects. No more bombs were found Wednesday.

On Tuesday, in an address to Parliament, Wijewardene said 'weakness' within Sri Lanka's security system had led to the failure to prevent the bombings.

'By now it has been established that the intelligence units were aware of this attack and a group of responsible people were informed about the impending attack,' Wijewardene said. 'However, this information has been circulated among only a few officials.'

In a live address to the nation late Tuesday, President Maithripala Sirisena said he also was kept in the dark on the intelligence about the planned attacks and vowed to 'take stern action' against officials who failed to share the information. He also pledged 'a complete restructuring' of the security forces.

Meanwhile, on Wednesday, Wijewardene also edged away from Tuesday comments that the bombings were retaliation for the March 15 mosque shootings in Christchurch, New Zealand, that killed 50 people. He told reporters Wednesday that the mosque attack may have been a motivation for the bombings, but that there was no direct evidence of that.

An Australian white supremacist was arrested in the Christchurch shootings.

While Sri Lanka's recent history has been rife with ethnic and sectarian conflict, the Easter Sunday attacks came as a shock.

Sri Lanka is dominated by Sinhalese Buddhists, but the country of 21 million also has a significant Tamil minority, most of whom are Hindu, Muslim or Christian.

Tamil Tiger rebels were known for staging suicide bombings during their 26-year civil war for independence, but religion had little role in that fighting. The Tigers were crushed by the government in 2009. Anti-Muslim bigotry fed by Buddhist nationalists has swept the country since the war ended but Sri Lanka has no history of Islamic militancy. Its small Christian community has seen only scattered incidents of harassment.