"While many details of the attacks are still emerging, we can confirm that several US citizens were among those killed," he said in a statement.
"The US Embassy is working tirelessly to provide all possible assistance to the American citizens affected by the attacks and their families."
The attacks, which police said left more than 450 people wounded in addition to at least 207 dead, constituted the worst act of violence since the end of Sri Lanka's civil war a decade ago.
The government imposed an indefinite nationwide curfew. There were no immediate claims of responsibility.
Over 200 killed as eight blasts rock Sri Lanka during Easter celebrations
Earlier reports citing Sri Lankan officials said the number of dead foreigners as between 27 and 35, including Americans, British and Dutch.
A White House statement said that "the United States condemns in the strongest terms the outrageous terrorist attacks in Sri Lanka."
Since the 26-year civil war in Sri Lanka ended in 2009, the country -- noted for its friendliness, natural beauty, its temples and mix of cultures -- had drawn increasing numbers of foreign tourists.
The Lonely Planet guide last year named it "the best country in the world to visit in 2019."
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