The US Embassy here has said that at the request of the Sri Lankan Government, US security experts were working closely with their Sri Lankan partners on "fulfilling short term, specific objectives" related to the recent attacks and to bringing the perpetrators to justice.

Nancy VanHorn, the spokesperson at the US Embassy in Colombo that the US believes active members of the attack group that carried out the terror attacks on Easter Sunday may still be at large.

"As the (US) Ambassador (Alaina Teplitz) has previously stated and as reflected in our travel advisory, the terrorist threat is ongoing," the spokesperson told the Colombo Gazette.

Sri Lankan authorities said they believed a little-known local militant Islamist group known as National Tawheed Jamath (NTJ) was behind the attack that killed 253 people and injured nearly 500 more. However, the NTJ has not claimed responsibility for the attacks, Sri Lanka's worst.

On Tuesday, the Islamic State terror group said it had carried out the attacks, and released video of men it says were the bombers, pledging allegiance to the group.

The attackers targetted three Catholic churches and three luxury hotels.

Ambassador Teplitz has said the attacks are the work of a few individuals and not of an entire community.

As Sri Lanka looks to the future and to implementing changes in security and communications procedures to prevent future atrocities, the US stands ready to assist with lessons learned from our own past tragedies and through our ongoing cooperation with local authorities, the embassy has said.

"These terrible attacks are the work of a few individuals and not of an entire community. Sri Lankans of all backgrounds and faiths have come together to condemn these atrocities. Unity is the most powerful answer to terrorism," the ambassador said recently.

Teplitz further underscored that "while we work together to bring the perpetrators of these heinous crimes to justice, we must remain vigilant in defending the democratic ideals that form the pillars of a strong society.

"And we must do so without destroying communities of peaceful, innocent people who share the faith of the attackers, but not their warped ideology," she said.

"We must always respect the wonderful diversity of Sri Lanka and fortify the culture of unity that the country needs to thrive," she said.

On April 26, the Department of State ordered the departure of all school-age family members of US government employees in Kindergarten through 12th grade. The Department also authorised the voluntary departure of non-emergency US government employees and family members from Sri Lanka.

"Terrorist groups continue plotting possible attacks in Sri Lanka," the State Department said, adding that terrorists may attack with little or no warning, targeting tourist spots, religious places other public areas.

According to Sri Lankan Foreign Ministry, one American national, two people holding US and UK nationalities were among 42 foreign nationals killed in the Easter Sunday attack.