At least three Muslims were killed and 75 people seriously injured in violence that erupted on Sunday between Buddhists and Muslims in southern Sri Lankan coastal towns, officials and residents said.
It was part of an increasing pattern of violence against Muslims in Sri Lanka since 2012. This has mirrored events in Myanmar, where there has been a surge of attacks by majority Buddhists against Muslims.
U.S. State Department spokeswoman Jan Psaki said Washington was concerned by inflammatory rhetoric that had incited the violence.
"We urge the Sri Lankan government to fulfill its obligations to protect religious minorities, including protecting all citizens and places of worship, conducting a full investigation into the violence and bringing those responsible to justice," she told a regular briefing.
"We also urge all sides to refrain from violence, exercise restraint and respect the rule of law."
Amnesty International said it was the worst outbreak of communal violence in Sri Lanka in years and there was a real risk of it spreading further. The rights group said the Sri Lankan authorities must act immediately to end the anti-Muslim violence and to rein in groups targeting religious minorities.
“Eyewitness reports that police have stood by and refused to intervene in the violence are very troubling and must also be investigated," Amnesty's Deputy Asia Pacific Director David Griffiths said in a statement.
"Security forces have a duty to protect the right of everyone to life and security regardless of their beliefs or identity.”
The clashes erupted on Sunday in Aluthgama and Beruwela, two Muslim-majority towns on the Sinhalese-dominated southern coast, during a protest march led by the hardline Buddhist groupBodu Bala Sena (BBS), or "Buddhist power force".
Many independent analysts say well-coordinated violence against Muslims and Christians appears to have tacit state backing as those involved in previous attacks have yet to be punished. The government denies any collusion.
A Reuters team in Aluthgama and Beruwela witnessed an uneasy calm and a heavy police presence, with Muslims worried for their safety, many of them sitting on the road in front of their ruined houses. Reuters reporters counted 16 houses gutted by fire.
Many residents said the police directly and indirectly helped the BBS organization. Police rejected the claim.
The BBS has said its members came under attack when they were protesting peacefully against an assault on a Buddhist monk by a Muslim youth three days earlier.
The Colombo government has been strongly criticized by the United States and other Western governments and the United Nations for alleged rights abuses in crushing the Tamil Tigers in a 2009 offensive.
The United Nations is launching an investigation into alleged war crimes. The government has said it will not cooperate with the inquiry and it has rejected Western calls for demilitarization in the former conflict zone.