Ranil Wickremesinghe said "it is possible" the co-ordinated attacks on churches and hotels were a response to the shootings in Christchurch last month, in which 50 people died.
The prime minister's comments on Tuesday afternoon were less definitive than those of minister of defence Ruwan Wijewardene, who had earlier told the country's parliament: "The initial investigation has revealed that this was in retaliation for the New Zealand mosque attack."
Mr Wijewardene, who did not say where his information came from, also said two domestic Islamist groups were responsible for the attacks: National Thaweed Jama'ut and Jammiyathul Millathu Ibrahim.
Mr Wickremesinghe's and Mr Wickremesinghe's comments came as the Islamic State group claimed responsibility for the attacks - but offered no evidence.
The extremist group said: “The perpetrators of the attack that targeted nationals of the countries of the coalitions and Christians in Sri Lanka before yesterday are fighters from the Islamic State.”
It offered no photographs or videos of attackers pledging their loyalty to the group. The group, which has lost all the territory it once held in Iraq and Syria, has made a series of unsupported claims of responsibility.
Meanwhile, prime minister Mr Wickremesinghe said the attacks could have been prevented, or at least reduced in scale, if intelligence reports had been passed on correctly.
He said some officials will likely lose their jobs over intelligence lapses surrounding the attack.
On Tuesday, the death toll increased to 321. About 500 people were injured.
Eight Brits were among the fatalities. On Tuesday, teenage brother and sister Daniel and Amelie Linsey were the latest victims to be named.
A team of specialist counter terrorism officers from Scotland Yard were being sent to Sri Lanka to aid the investigation and help families of the British victims.
On Tuesday, Sri Lanka declared a state of emergency, giving the country's military war-time powers and a wider berth to detain and arrest suspects.
Forty have been arrested, including the driver of a van allegedly used by the suicide bombers and the owner of a house where some of them lived.
Authorities also stepped up security measures, with police issuing orders for anyone parking a car on the street to display a note with their phone number on the windscreen.