* Six children among the 15 bodies found after shootout

* Bomb-making material, gelignite sticks, ball bearings discovered

* Authorities warn of possible further attacks

The bodies of 15 people, including six children, were discovered at the site of a fierce overnight gun battle on the east coast of Sri Lanka, a military spokesman said on Saturday, six days after suicide bombers killed more than 250 people.

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

A police spokesman said that three suspected suicide bombers were among the 15 dead after the shoot out.

One child caught in the crossfire was admitted to hospital.

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 domestic Islamist group National Towheed Jama'at (NTJ), which has been blamed for last Sunday's attacks.

Bomb-making materials, dozens of gelignite sticks and thousands of metal balls were found in a search of a separate house in the same area, the military said.

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, 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. Twenty were arrested in the past 24 hours alone, they said.

AUTHORITIES WARN OF MORE ATTACKS

Islamic State 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 Islamic State flag and declaring their loyalty to its leader, Abu Bakr Al-Baghdadi.

Muslims in Sri Lanka were urged to pray at home on Friday after the State Intelligence Services warned of possible car bomb attacks, amid fears of retaliatory violence.

Fears of retaliatory sectarian violence have already caused Muslim communities to flee their homes amid bomb scares, lockdowns and security sweeps.

The US Embassy in Sri Lanka urged its citizens to avoid places of worship over the weekend after authorities reported there could be more attacks targeting religious centres.

Archbishop of Colombo Cardinal Malcolm Ranjith told reporters he had seen a leaked internal security document warning of further attacks on churches and there would be no Catholic masses this Sunday anywhere on the island.

Nearly 10,000 soldiers were deployed across the Indian Ocean island state to carry out searches and provide security for religious centres, the military said.

Authorities have so far focused their investigations on international links to two domestic groups they believe carried out the attacks, NTJ and Jammiyathul Millathu Ibrahim.

INTELLIGENCE FAILURE

Officials have acknowledged a major lapse in not widely sharing intelligence warnings from India of possible attacks.

President Maithripala Sirisena said on Friday that top defence and police chiefs had not shared information with him about the impending attacks.

He blamed Prime Minister Ranil Wickremesinghe's government for weakening the intelligence system by focusing on prosecution of military officers over alleged war crimes during a decades-long civil war with Tamil separatists that ended in 2009.

Sirisena fired Wickremesinghe in October over political differences, only to reinstate him weeks later under pressure from the Supreme Court.

Opposing factions aligned to Wickremesinghe and Sirisena have often refused to communicate with each other and blame any setbacks on their opponents, government sources say.

The Easter Sunday bombings shattered the relative calm that had existed in Buddhist-majority Sri Lanka since the civil war against mostly Hindu ethnic Tamil separatists ended.

Sri Lanka's 22 million people include minority Christians, Muslims and Hindus. Until now, Christians had largely managed to avoid the worst of the island's conflict and communal tensions.

Most of the victims were Sri Lankans, although authorities said at least 40 foreigners were also killed, many of them tourists sitting down to breakfast at top-end hotels when the bombers struck.

They included British, US, Australian, Turkish, Indian, Chinese, Danish, Dutch and Portuguese nationals. Britain warned its nationals this week to avoid Sri Lanka unless it was absolutely necessary.