The US State Department said in a revised travel advisory 'terrorist groups' were continuing to plot possible attacks in Sri Lanka after 290 people were killed and about 500 wounded in blasts in churches and luxury hotels on Sunday.
"Terrorists may attack with little or no warning," it said in the revised warning, which was dated Sunday U.S. time.
It said possible targets included tourist locations, transportation hubs, shopping malls, hotels, places of worship, airports and other public areas.
U.S. President Donald Trump called Prime Minister Ranil Wickremesinghe to pledge U.S. support in bringing the perpetrators to justice, a White House spokesman said.
"A terrible, terrible thing. Unthinkable," Trump later told reporters at a White House event. "We are working with Sri Lanka."
Pope Francis deplored the attacks and called for universal condemnation of what he said were "terrorist acts, inhuman acts" that could never be justified.
Buddhist-majority Sri Lanka was at war for decades with ethnic minority Tamil separatists, most of them Hindu, but violence had largely ended since the government victory in the civil war 10 years ago. The country's 22 million people include minority Christians, Muslims and Hindus.
Most of the dead and wounded were Sri Lankans although government officials said 32 foreigners were killed, including British, U.S., Australian, Turkish, Indian, Chinese, Danish, Dutch and Portuguese nationals.