WASHINGTON: The US State Department said in a revised travel advisory "terrorist groups" were continuing to plot possible attacks in Sri Lanka after 290 people were killed and about 500 wounded in blasts in churches and luxury hotels on Sunday (Apr 21).
"Terrorists may attack with little or no warning," it said in the revised warning, which was dated Sunday US time.
It said possible targets included tourist locations, transportation hubs, shopping malls, hotels, places of worship, airports and other public areas.
US Secretary of State Mike Pompeo said "several" Americans were killed, as the latest US travel advisory urged "increased caution".
READ: 'People screaming, dead bodies all around': Survivor recalls chaos during deadly Sri Lanka blasts
There was still no claim of responsibility for the Easter Sunday attacks on two churches and four hotels in and around Colombo, the capital of predominantly Buddhist Sri Lanka, and a third church on the South Asian nation's northeast coast.
Some Sri Lankans asked how the attacks could have happened after it emerged that the government had been warned about possible blasts.
Sri Lanka's police chief Pujuth Jayasundara had issued an intelligence alert to top officers on Apr 11, warning that suicide bombers from a group called National Thowheeth Jama'ath (NTJ) planned to hit "prominent churches".
The NTJ is a radical Muslim group in Sri Lanka that was linked last year to the vandalisation of Buddhist statues.
Prime Minister Ranil Wickremesinghe acknowledged on Sunday that the government had some prior information about possible attacks on churches involving a little-known Islamist group but said ministers had not been told.
He added that an investigation would look into "why adequate precautions were not taken".
Authorities will also look into whether the attackers had "overseas links".
The island-wide curfew imposed by the government was lifted early on Monday, although there was uncharacteristically thin traffic in the normally bustling capital. The government has ordered a two-day holiday and schools and the Colombo stock exchange were closed
Soldiers armed with automatic weapons stood guard outside major hotels and the World Trade Centre in the business district, where the four hotels were targeted on Sunday, according to a Reuters witness.
Scores of people who were stranded overnight at the main airport began making their way home as restrictions were lifted.
The government also blocked access to social media and messaging sites, including Facebook and WhatsApp, making information hard to gather.
The attacks were the worst ever carried out against Sri Lanka's small Christian minority, who make up just seven per cent of the 21 million population.
At least 37 foreigners were among the dead, the foreign ministry said.
The government said three Indians, three Britons, two from Turkey and one Portuguese had died, as well as two people holding both British and US passports.
"Additionally, while nine foreign nationals are reported missing, there are 25 unidentified bodies believed to be of foreigners," the foreign ministry said.
On Apr 25, the Sri Lankan government revised the death toll to 253.