Several Americans and Britons were among the more than 200 people killed Sunday in a series of bombings of hotels and churches in Sri Lanka. (Reuters photo)

Several Americans were among the more than 200 people killed Sunday in a series of bombings of hotels and churches in Sri Lanka, Secretary of State Mike Pompeo said.

"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.

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."

Britons were also among those killed in the deadly Easter Sunday blasts in Sri Lanka, the Foreign Office in London said.

“We can confirm that British nationals were among those killed in today’s horrific attacks in Sri Lanka,” a Foreign and Commonwealth Office spokeswoman told AFP.

“Our staff are supporting the relatives of the victims and are continuing to work with the relevant authorities to obtain further information.

“We extend our deepest condolences to those who have lost loved ones and all those affected by this senseless tragedy.”

The spokeswoman did not give a figure for the Britons killed, but a government ministry source said London was aware of at least five British fatalities.

A series of eight devastating bomb blasts ripped through high-end hotels and churches holding Easter services, killing more than 200 people, including dozens of foreigners.

James Dauris, Britain’s High Commissioner to Sri Lanka, was attending an Easter Day church service in Colombo that was cut short by the attacks. He visited UK nationals in hospital in the capital and called the attacks “evil.”

British Prime Minister Theresa May condemned the blasts, saying on Twitter: “The acts of violence against churches and hotels in Sri Lanka are truly appalling, and my deepest sympathies go out to all of those affected at this tragic time.

“We must stand together to make sure that no one should ever have to practise their faith in fear.”

In her Easter message released just hours before the blasts, May – the daughter of an Anglican vicar – offered her support to Christians around the world who face “huge danger” because of their faith.

She said: “Churches have been attacked. Christians murdered. Families forced to flee their homes.

“That is why the government has launched a global review into the persecution of Christians.

“We must stand up for the right of everyone, no matter what their religion, to practise their faith in peace.”

Britain’s Foreign Secretary Jeremy Hunt also condemned the Sir Lanka blasts.

He tweeted: “I’m deeply shocked and saddened by the horrifying attacks on churches and hotels in Sri Lanka.

“To target those gathered for worship on Easter Sunday is particularly wicked.” -- AFP