Sri Lanka's prime minister has said he believes the attacks on Easter Sunday which claimed the lives of over 320 people had links to the so-called Islamic State group. 

The militant group has claimed responsibility for the series of the bombings, which were carried out at three churches and ffour hotels.

Ranil Wickremesinghe told reporters that the government's security agencies were monitoring Sri Lankans who had joined Islamic State and returned home.

"We will be following up on IS claims, we believe there maybe links," he said.

It comes as the first funerals of the victims took place today, and the country has begun a day of national mourning with three minutes of silence to honour the people who died in the blasts. 

At least 45 children were killed in the attacks, according to UNICEF, which said that others who were wounded are "fighting for their lives".

Flags were lowered to half mast on government buildings, and people bowed their heads and reflected silently on the violence that has caused international outrage.

The silence began at 8.30am (3am Irish time), the time that the first of six bombs detonated on Sunday morning, unleashing carnage at high-end hotels and churches packed with Easter worshippers.

At St Anthony's Shrine in Colombo, where scores died as they gathered for Easter Sunday prayers, a crowd of a few dozen people held candles and prayed silently.

Some of them struggled to hold back tears and as the three minutes drew to a close, the crowd began to chant prayers.

Police said that 40 people were now under arrest, including a Syrian national, over the suicide bomb attacks - the worst atrocity since Sri Lanka's civil war ended a decade ago.

The attacks were also the worst ever against the country's small Christian minority, who make up just 7% of its population of 21 million.

The FBI is assisting Sri Lankan authorities with their investigation of the blasts, a spokeswoman for the US law enforcement agency has said.

The Washington Post earlier had reported that the FBI had offered expertise to test evidence and that analysts were scouring databases for information regarding the attacks.

Investigators are now hunting for clues on whether the local Islamist group named as the chief suspect - National Thowheeth Jama'ath (NTJ) - received "international support", said cabinet minister and government spokesman Rajitha Senaratne.

The spokesman added that it was not possible for such "a small organisation" to carry out such well coordinated suicide strikes.

Funeral ribbons hang across a road leading to St Anthony's Shrine in Colombo, Sri Lanka

President Maithripala Sirisena's office said there was intelligence that "international terror groups" were "behind local terrorists" and that he would seek foreign help to investigate.

A Sri Lankan official said the initial investigation has revealed that this was in retaliation for the attacks on two mosques in Christchurch, New Zealand in which 50 people were killed.

He said: "It was done by National Thawheed Jama'ut along with JMI", referring to another local group, Jammiyathul MillathuIbrahim.

However, Islamic State today claimed responsibility for the bombings, the group's AMAQ news agency said.

The group did not give evidence for its claim.

The state of emergency, which gave police and the military special powers to counter militant strikes, came into force at midnight. Suspects can be detained without a court order.

A second straight night-time curfew was also put in place, but lifted before dawn.

Officials are also investigating why more precautions were not taken after an 11 April warning from Sri Lanka's police that a "foreign intelligence agency" had reported the NTJ planned suicide attacks on churches.

Mr Senaratne said the warning was not passed on to Prime Minister Ranil Wickremesinghe or other top ministers.

Tension remains high and security heavy after a bomb discovered by police on Monday near one of the targeted churches blew up before police could defuse it. Although there was a powerful blast, no injuries were reported.

Police also found 87 bomb detonators at a Colombo bus station.

Details have begun to emerge about some of the foreigners killed in the blasts. 

The United States reported at least four Americans killed, including a child, and the Netherlands raised their toll to three.

A Danish billionaire lost three of his children in the attacks, a spokesman for his company said.

Eight Britons, eight Indians and nationals from Turkey, Australia, France, Japan and Portugal, were also killed, according to Sri Lankan officials and foreign governments.

The suicide bombers hit three Colombo luxury hotels popular with foreign tourists - the Cinnamon Grand, the Shangri-La and the Kingsbury - and three churches: two in the Colombo region and one in the eastern city of Batticaloa.

Two additional blasts were triggered as security forces carried out raids searching for suspects.

Interpol said it was deploying investigators and specialists to Sri Lanka, and the US State Department warned of possible further attacks in a travel advisory.