Updated at 12.05pm

The Sri Lankan government believes a local Islamist extremist group called National Thowheeth Jama'ath (NTJ) was behind deadly suicide bomb attacks that killed nearly 300 people, government spokesman Rajitha Senaratne said Monday.

Senaratne, who is also a cabinet minister, added that the government was investigating whether the group had "international support."

Later on Monday, the government declared a state of emergency from midnight.

Documents seen by AFP show Sri Lanka's police chief issued a warning on April 11, saying that a "foreign intelligence agency" had reported NTJ was planning attacks on churches and the Indian high commission.

In the meantime, the US State Department has warned of possible terrorist attacks in a revised travel advisory for Sri Lanka, after a series of deadly blasts killed nearly 300 people - including Americans - in the island nation on Easter Sunday.

Sunday's attacks saw suicide bombers target churches during Easter services and high-end hotels in the capital Colombo.

Secretary of State Mike Pompeo said "several" Americans were killed - the worst violence to hit Sri Lanka since a long and bloody civil war ended a decade ago.

The latest US travel advisory urged "increased caution."

"Terrorist groups continue plotting possible attacks in Sri Lanka," it said.

"Terrorists may attack with little or no warning, targeting tourist locations, transportation hubs, markets/shopping malls" and other public locations like hotels, clubs, restaurants, and places of worship.

Travel advisories from other nations like Australia and Ireland also suggest citizens exercise caution while in Sri Lanka.

Authorities have said citizens of India, China, Denmark, the Netherlands, Japan, Portugal, Britain, and Turkey died in the attacks - including three of the four children of Asos tycoon Anders Holch Povlsen, and two relatives from an Australian family.

The US embassy in Colombo said it would remain closed to the public on Monday and Tuesday, but will continue to provide citizen services.

Security has been ramped up across Sri Lanka as security forces hunt for suspects, with 24 people arrested so far. Wary of sparking ethnic and religious tensions, the government has given few details about those detained.

Authorities have said, however, that they are looking into whether the attackers had any "overseas" links.

Travel guide book publisher Lonely Planet in October named Sri Lanka as its top destination for 2019, describing it as "the island opening up to new travellers like never before".

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