Authorities in Sri Lanka have banned women from wearing face veils under an emergency law put in place after deadly Easter Sunday attacks by Islamist militants.

Authorities said the measures would help security forces to identify people as a hunt for any remaining attackers and their support network continues across the Indian Ocean island.

But there are concerns within the Muslim community that a prolonged ban could fuel tensions in the religiously diverse nation that emerged from a civil war with ethnic minority Tamil separatists a decade ago.

Officials have warned that the militants behind the 21 April suicide bombings on hotels and churches that killed over 250 people were planning more attacks, using a van and bombers disguised in military uniforms.

"It is a presidential order to ban any dress covering faces with immediate effect," a spokesman for President Maithripala Sirisena, told Reuters.

Separately, Prime Minister Ranil Wickremesinghe, who is feuding with Mr Sirisena, issued a statement saying he had asked the justice minister to draft regulations to ban the burqa.

The All Ceylon Jamiyyathul Ulama (ACJU), the top body of Islamic scholars in Sri Lanka, said they supported a short-term ban on security grounds, but opposed any attempt to legislate against burqas.

"We have given guidance to the Muslim women to not to cover their faces in this emergency situation," an ACJU representative said after the scholars asked the government to drop plans for a law against the burqa and niqab.

"If you make it a law, people will become emotional and this will bring another bad impact ... it is their religious right," he said.

About 9.7% of Sri Lanka's roughly 22 million people are Muslim. Only a small minority of women, usually in Muslim areas, fully hide their faces.

Human Rights Watch condemned the ban.

"That needless restriction means that Muslim women whose practice leads them to cover up now won't be able to leave home," the group's executive director tweeted.

Wife, daughter of bombings mastermind will survive blast - hospital

The wife and daughter of the suspected mastermind of suicide bombings escaped with minor injuries from an explosion at a safe house where they had been holed up, hospital staff and police have said.

Mohamed Zahran Hashim was the ringleader of a group who blew themselves up in churches and hotels.

His father and two of his brothers were killed when security forces stormed their safe house on the east coast of the country on Friday night, police and a relative said.

The Sri Lankan military said at least 15 people were killed during a gun battle there, and Islamic State said on Sunday that three of its members detonated explosive vests at the end of the clash.

The only two survivors of the blasts were Zahran's wife, Abdul Cader Fathima Sadia, and his four-year-old daughter, Mohamed Zahran Rusaina, who were pulled from the wreckage on Saturday morning.

Sadia is a suspect in the Easter Sunday bombings, however she is not under arrest, according to the hospital and local police officers.