COLOMBO (REUTERS) - Muslims in Sri Lanka were urged to pray at home on Friday (April 26) and not attend mosques or churches after the state intelligence services warned of possible car bomb attacks, amid fears of retaliatory violence for the Easter Sunday bombings.

The United States embassy in Sri Lanka also urged its citizens to avoid places of worship over the coming weekend after the authorities reported there could be more attacks targeting religious centres.

Britain also warned its nationals on Thursday to avoid Sri Lanka unless it was absolutely necessary because there could be more attacks.

Sri Lanka remains on edge after suicide bombing attacks on three churches and four hotels that killed 253 people and wounded about 500. The attacks have been claimed by the extremist Islamic State in Iraq and Syria (ISIS) group.

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

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

The All Ceylon Jamiyathul Ullama, Sri Lanka's main Islamic religious body, urged Muslims to conduct prayers at home on Friday in case "there is a need to protect family and properties".

Cardinal Malcolm Ranjith also appealed to priests not to conduct mass at churches until further notice.

"Security is important," he said.

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

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

The Sri Lankan government said there were nine homegrown, well-educated suicide bombers, eight of whom had been identified. One was a woman.

Sri Lankan President Maithripala Sirisena said on Friday police are looking for 140 people believed to have links with ISIS.

He said some Sri Lankan youths had been involved with ISIS since 2013.

He said information uncovered so far suggested there were 140 people in Sri Lanka involved in ISIS activities.

"Police are looking to arrest them," Sirisena said.

The authorities have focused their investigations on international links to two domestic Islamist groups - National Thowheeth Jama'ath and Jammiyathul Millathu Ibrahim - they believe carried out the attacks.

Government officials have acknowledged a major lapse in not widely sharing an intelligence warning from India before the attacks.

Mr Sirisena said top defence and police chiefs had not shared information with him about the impending attacks. Defence Secretary Hemasiri Fernando resigned over the failure to prevent the attacks.

"The police chief said he will resign now," Mr Sirisena said.

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

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

Opposing factions aligned to Mr Wickremesinghe and Mr 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 a civil war against mostly Hindu ethnic Tamil separatists ended 10 years ago.

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 the authorities said at least 38 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.