Officials inspect the damaged St. Sebastian's Church after multiple explosions targeting churches and hotels across Sri Lanka on April 21, 2019 in Negombo, north of Colombo, Sri Lanka.

Devastating Easter bombings in Sri Lanka were retaliation for recent deadly attacks on mosques in New Zealand, a Sri Lankan official said on Tuesday, adding that two domestic Islamist groups were believed to have been behind Sunday's blasts.

No group has claimed responsibility for the coordinated attacks, which officials said were carried out by at least seven suicide bombers, on three churches and four hotels. The toll rose to 321 dead with about 500 people wounded.

"The initial investigation has revealed that this was in retaliation for the New Zealand mosque attack," junior minister for defence Ruwan Wijewardene told parliament.

He did not elaborate on why authorities believed there was a link to the killing of 50 people at two mosques in the New Zealand city of Christchurch during Friday prayers on March 15. A lone gunman carried out those attacks.

Wijewardene said two Sri Lankan Islamist groups - the National Thawheed Jama'ut and Jammiyathul Millathu Ibrahim - were responsible for the blasts early on Sunday during Easter services and as high-end hotels served breakfast.

U.S. intelligence sources said the attacks carried some of the hallmarks of the Islamic State militant group, although they were cautious because it had not claimed responsibility.

Islamic State is usually quick to claim responsibility for, or links to, attacks against foreign targets or religious groups, whether it was involved or not, they said.

Prime Minister Ranil Wickremesinghe told parliament investigators were looking into foreign links.

Earlier on Tuesday, Sri Lankan government and military sources said a Syrian had been detained among 40 people being questioned over the bombs.

"He was arrested after the interrogation of local suspects," one of the sources said, referring to the unidentified Syrian.