Sri Lankan authorities warn of more attacks in the capital city Colombo. (Getty Images)

Sri Lankan authorities on Friday (3) have warned that Islamist extremists, who carried out the country’s worst terror bombings on Easter Sunday could be planning more attacks in the capital Colombo.

The security has been further beefed up following another leakage of a police internal circular which warned that some of the entry bridges to the city of Colombo and an overhead bridge north Colombo would be blasted by around May 6 by the same terror group which carried out the Easter Sunday attacks.

“Extensive cordon and search operations of the army and sister services across the country with assistance of the police are continuing in search of terrorists, hideouts, explosives, weapons and other warlike items… with the deployment of more and more troops as required,” the Sri Lanka Army said in a statement.

Sri Lankan authorities have admitted that there was a failure to act on advance intelligence warnings of the deadly Easter Sunday attacks against churches and luxury hotels that killed 253 people and injured nearly 500 others.

President Sirisena said a friendly neighbour provided additional information but senior officials did not share the intelligence with him.

Prime minister Ranil Wickremesinghe also said he was not privy to the intelligence reports.

Authorities earlier said they believed a little-known local militant Islamist group known as National Tawheed Jamath (NTJ) was behind the attack. However, the NTJ has not claimed responsibility for the attacks, Sri Lanka’s worst.

The Islamic State terror group said it had carried out the attacks, and released video of men it says were the bombers.

The security has been conducting raids on radical Islamist groups and individuals with links to the bombings.