Secretary of State Mike Pompeo on Monday vowed that the US will continue fighting "radical Islamic terror" and the "evil human beings" perpetrating it as he condemned Sri Lanka's worst terror attacks that claimed 290 lives.

Pompeo has said America will continue fighting "radical Islamic terror", which he said remained a threat following the attacks in Sri Lanka.

"Sadly, this evil exists in the world," he told reporters on Monday. "This is America's fight, too."

Pompeo said that he had spoken to Sri Lankan Prime Minister Ranil Wickremesinghe over the telephone.

He earlier tweeted to condemn the "brutal" attacks, adding that the US offered its "deepest condolences".

"Radical Islamic terror remains a threat. We are continuing to do real work against these evil human beings," Pompeo told reporters.

Asked if the threat showed the continued risks from the Islamic State group, Pompeo said the US had succeeded in destroying the movement's self-styled caliphate in Syria but needed to "remain active and vigilant" around the world.

"Sadly, this evil exists in the world," he said.

Meanwhile, Dieter Kowalski from Colorado has been identified as one of the Americans killed in the attacks in Colombo, according to Pearson, the education publishing company that Kowalski worked for, CNN reported.

"Dieter had just arrived at his hotel, where many of our colleagues have stayed over the years, when he was killed in an explosion," Pearson CEO John Fallon said in a statement.

Dieter's brother Derrick Kowalski confirmed his brother's death on Facebook, writing, "as we know that Dieter saw his friends as family, we would like to share our grief over this tragic incident.

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