A blast hit near a church in Sri Lanka's capital on Monday as police tried to defuse a new bomb found by the site, a police spokesman said.
There was no immediate information on injuries in the blast, or how large it was. The explosion happened around 50 metres from the St Anthony's Shrine, one of three churches targeted in a string of suicide bombs on Sunday that killed nearly 300 people.
The Sri Lankan government believes a local Islamist extremist group called the National Thowheeth Jama'ath (NTJ) was behind the deadly suicide bomb attacks, government spokesman Rajitha Senaratne said Monday.
Senaratne, who is also a cabinet minister, added that the government was investigating whether the group had "international support".
"We don't see that only a small organisation in this country can do all that," he said.
"We are now investigating the international support for them, and their other links, how they produced the suicide bombers here, and how they produced bombs like this."
Documents seen by AFP show Sri Lanka's police chief issued a warning on 11 April, saying that a "foreign intelligence agency" had reported NTJ was planning attacks on churches and the Indian high commission.
Not much is known about the NTJ, a radical Muslim group that has been linked to the vandalising of Buddhist statues.
A police source told AFP that all 24 people in custody in connection with the attacks belong to an "extremist" group, but did not specify further.