Sri Lanka said Monday it believed a local Islamist extremist group was behind deadly suicide bomb blasts that killed nearly 300 people as it announced a national state of emergency beginning midnight.

Government spokesman Rajitha Senaratne said investigators were looking at whether the National Thowheeth Jama'ath (NTJ) group had "international support" for the deadly Easter Sunday attacks on churches and luxury hotels.

Wary of stirring ethnic and religious tensions, officials have provided few details about 24 people arrested since the attacks.

Not much is known about the NTJ, but documents seen by AFP show Sri Lanka's police chief issued a warning on April 11, saying a "foreign intelligence agency" had reported the group was planning attacks on churches and the Indian high commission.

The group has previously been linked to the vandalising of Buddhist statues.

The death toll from Sunday's attacks rose dramatically Monday to 290 -- including dozens of foreigners -- in the country's worst attacks for over a decade.

More than 500 people were injured in the assault that saw suicide bombers hit three high-end hotels popular with foreign tourists, and three churches, unleashing carnage in Colombo and beyond.

Two additional blasts were triggered as security forces carried out raids searching for suspects.

As tension remained high, police on Monday said they had found 87 bomb detonators -- 12 of them scattered on the ground at a bus station and another 75 in a nearby garbage dump.

- State of emergency -

The president's office said a state of emergency "limited to counter terrorism regulations only" would be introduced from midnight Monday (1830 GMT).

"This is being done to allow the police and the three forces to ensure public security," the statement said, referring to the army, navy and air force.

The government information department said a new curfew would run from 8:00 pm (1430 GMT) on Monday until 4:00 am on Tuesday.

The US State Department, meanwhile, warned of further attacks in a revised travel advisory, urging increased caution and adding: "Terrorist groups continue plotting possible attacks in Sri Lanka."

The attacks were the worst ever carried out against Sri Lanka's small Christian minority, who make up just seven percent of the 21 million population.

At least 37 foreigners were among the dead, citizens of India, Britain, Turkey, Australia, Japan and Portugal, as well as a dual US-British passport holder.

The churches targeted included St Sebastian's in Negombo, north of the capital, which was surrounded by security forces on Monday.

Dozens of people were killed at the church, including friends of 16-year-old Primasha Fernando, who was at her home nearby when the suicide bomber struck.

"When I got to the church there were people crying and screaming," she told AFP.

"I saw bodies everywhere," she added in tears. "I saw parents carrying their dead babies. I saw dead people who had hair but didn't have faces anymore."

"Hands and legs were separate from bodies. There was blood everywhere too. The smell was so strong it made me feel sick."

The church's roof was largely blown out and pews splintered. The floor was strewn with roof tiles and shards of glass.

- Memories of civil war -

Ethnic and religious violence has plagued Sri Lanka for decades, with a 37-year conflict with Tamil rebels followed by an upswing in recent years in clashes between the Buddhist majority and Muslims.

Dilip Fernando, who could not get into St Sebastian's because the church was already packed when he arrived, said the Christian community would not be intimidated.

"We are not afraid. We won't let the terrorists win, no way," the 66-year-old told AFP outside the devastated building.

Two leading Muslim groups issued statements condemning the attacks, with the All Ceylon Jamiyaathuul Ulama, a council of Muslim theologians, urging the "maximum punishment for everyone involved in these dastardly acts."

For many, the blasts stirred painful memories of Sri Lanka's civil war, when bomb attacks were a frequent occurrence.

"The string of blasts brings back memories of the time when we were afraid to go in buses or trains because of parcel bombs," said Malathi Wickrama, a street sweeper in Colombo.

The attacks drew global condemnation, including from US President Donald Trump and the pope.

The archbishop of Colombo, Malcolm Ranjith, described the attackers as "animals" and called on authorities to "punish them mercilessly".

The attacks drew global condemnation, including from US President Donald Trump and the pope.

Premier Wickremesinghe urged people to "hold our unity as Sri Lankans" and pledged to "wipe out this menace once and for all".

The archbishop of Colombo, Malcolm Ranjith, described the attackers as "animals" and called on authorities to "punish them mercilessly".