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The death toll from a string of deadly suicide bomb attacks in Sri Lanka has risen to 310, with several people dying of their injuries overnight, a police spokesman said on Tuesday, Channel NewsAsia reports.

Around 500 people were wounded in the blasts, Ruwan Gunasekera said in a statement.

The update came as the country began a day of national mourning with three minutes of silence.

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 local 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.

Authorities said 40 people were now under arrest in connection with the attacks, which Sri Lanka's government has blamed on a previously little-known local militant group, National Thowheeth Jama'ath.

Those arrested are mainly Sri Lankans, although Gunasekera said police were investigating whether foreigners were involved in the attacks carried out by seven suicide bombers.

Cabinet minister and government spokesman Rajitha Senaratne said investigators are now hunting for clues on whether the group received "international support".

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

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 state of emergency has been declared which gives police extensive powers to detain and interrogate suspects without court orders. An overnight curfew was also put into effect.

The declaration came after nerves were frayed even further in the seaside capital Colombo when explosives went off on Monday near one of the churches hit in Sunday's attacks while bomb squad officers were working to defuse a device.