More than 300 people killed in suicide bomb blasts in Sri Lanka. (File)

Highlights Lanka minister said it was in retaliation to Christchurch mosque attacks

50 people were killed in shooting attacks on two mosques in New Zealand

Over 300 people died and hundreds injured in Sri Lanka serial blasts

An initial probe into deadly suicide bomb attacks in Sri Lanka that killed more than 300 people shows it was "retaliation for Christchurch," the country's deputy defence minister said Tuesday.

"The preliminary investigations have revealed that what happened in Sri Lanka (on Sunday) was in retaliation for the attack against Muslims in Christchurch," state minister of defence Ruwan Wijewardene told parliament.

Fifty people were killed in shooting attacks on two mosques in the New Zealand city of Christchurch on March 15.

Sri Lanka fell silent for three minutes Tuesday on a day of national mourning to honour those killed.

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 8:30 am (0300 GMT), the time that the first of six bombs detonated on Sunday morning, unleashing carnage at high-end hotels and churches packed with Easter worshippers.

Shortly after the silence was observed, a police spokesman said the death toll had risen to 310, with several people dying of their injuries overnight.

The first memorial services for the victims, among them dozens of foreigners, were being held Tuesday, hours after the government imposed a state of emergency and said an ISIS group was behind the violence.

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

And at St Sebastian's Church in Negombo, north of the capital, an elderly man wept uncontrollably by the coffin bearing the body of his wife.

Police said that 40 people were now under arrest 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 seven percent of the 21 million population.

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.

He said it was not possible for such "a small organisation" to carry out such well co-ordinated suicide strikes.

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

The state of emergency, which gave police and the military special powers to counter militant strikes, came into force at midnight (1830 GMT Monday). Suspects can be detained without a court order.