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Sri Lanka's military launched a major hunt on Saturday for remnants of an Islamist group which carried out the Easter suicide bombings that killed 258 people, officials said.

Several Colombo suburbs were targeted by troops using emergency powers on arrests and detentions adopted after the April 21 attacks.

"Special cordon-and-search operations are under way in three areas just outside Colombo," a military official told reporters.

Similar operations were also carried out in the country's north-west, where anti-Muslim riots this month left one man dead and hundreds of Muslim-owned shops, homes and mosques destroyed.

A second overnight curfew imposed nationwide ensured there was no repeat of Monday's violence against the Muslim minority.

Amnesty International have called on authorities in Sri Lanka to do more to protect Muslims.

"The Sri Lankan authorities must protect the country's Muslim minority as it is being targeted by mobs in horrific attacks on their homes, mosques and businesses in the wake of the Easter Sunday massacre," said Amnesty's South Asia Researcher, Thyagi Ruwanpathirana.

Read more: Fake news rampant after Sri Lanka attacks despite social media ban

President Maithripala Sirisena on Wednesday extended for one month the 30-day state of emergency imposed after the suicide bombings.

Security forces have arrested scores of suspects in connection with the bombings and over what appeared to be organised violence against the island's Muslim minority.

Sirisena said the move was to maintain "public security", with the country still on edge after the attacks on three hotels and three churches that were blamed on a local jihadi group, the National Thowheeth Jama'ath (NTJ).

The Islamic State group has also claimed a role in the attacks.

Christians make up 7.6 percent and Muslims 10 percent of Sri Lanka's population of 21 million, the majority being Buddhist.

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