Islamist extremists in Sri Lanka are believed to be planning attacks on bridges in Colombo, the authorities said, as they warned that several conspirators in the Easter Sunday bombings which killed 257 people were still at large.

Police confirmed they had instructed stations around Colombo to deploy additional officers and asked the navy to deploy more vessels on rivers, following leaks of police intelligence reports warning that bridges into the capital were at risk of attack.

Sri Lanka's military has also set up a special command centre to coordinate anti-Islamist operations, while the army said more troops have been deployed for search operations.

Additional troops conducted searches overnight and seized explosives and weapons from several locations, although these were from criminal groups and not extremists, official sources said.

"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 war-like items... with the deployment of more and more troops as required," the army said in a statement.

The authorities had information about a small group of radicals who may be trying to stage more strikes, said Health Minister and government spokesman Rajitha Senaratne, who added that the crackdown on extremists after the Easter bombings had been largely successful.

"You can't say the threat is over, but the situation is well under control... better than what we expected."

But the government was also still on the hunt for "four terrorists" involved in the Easter attacks who were still at large, Mr Senaratne told AFP in an interview on Thursday evening.

The Catholic Church announced on Thursday that it had called off the resumption of Sunday servicesfollowing information of a "specific threat" against two of their locations just outside the capital.

Catholic schools, which were due to reopen after an extended Easter holiday, will now be shut indefinitely.

Public schools are due to reopen on Monday under tight security.