The gun battle between troops and suspected Islamist militants erupted on Friday evening in Sainthamaruthu, Ampara, to the south of the town of Batticaloa, site of one of the Easter Sunday blasts at luxury hotels and churches.

The police and military had earlier said four gunmen and a civilian had been killed in the shootout. The 15 were found in the morning during clearance operations.

Military spokesman Sumith Atapattu said in a statement that as troops headed towards the safe house three explosions were triggered and gunfire began.

“Troops retaliated and raided the safe house where a large cache of explosives had been stored,” he said in a statement.

He said the militants were suspected members of the National Towheed Jama’at (NTJ), which has been blamed for last Sunday’s attacks.

The government has said nine homegrown, well-educated suicide bombers carried out the Easter Sunday attacks, eight of whom had been identified. One was a woman.

Police said on Friday they were trying to track down 140 people they believe have links with Islamic State (IS), which claimed responsibility for the suicide bombings.

Police have detained at least 76 people, including foreigners from Syria and Egypt, in their investigations so far.

IS provided no evidence to back its claim that it was behind the attacks.

If true, it would be one of the worst attacks carried out by the group outside Iraq and Syria.

The extremist group released a video on Tuesday showing eight men, all but one with their faces covered, standing under a black IS flag and declaring their loyalty to its leader, Abu Bakr Al-Baghdadi.