The United Nations Special Rapporteur on Myanmar's human rights said he had received reports of "state involvement" in some of the recent violence between Buddhists and Muslims in the former army-ruled nation.
At least 40 people have been killed and mosques burned in several towns in central Myanmar since fresh sectarian strife erupted on March 20, prompting the government to impose emergency rule and curfews in some areas.
"I have received reports of state involvement in some of the acts of violence," Tomas Ojea Quintana said.
He also pointed to "instances where the military, police and other civilian law enforcement forces have been standing by while atrocities have been committed before their very eyes, including by well organised ultra-nationalist Buddhist mobs".
"This may indicate direct involvement by some sections of the state or implicit collusion and support for such actions."
Quintana also said he received information indicating that the military and police may be arbitrarily detaining people based on religious and ethnic profiling.
"The military and police must now be held to account for human rights violations committed against ethnic and religious minorities," he said.
Quintana also called on the government to take "immediate action to stop the violence from spreading to other parts of the country and undermining the reform process."
"This includes stemming campaigns of discrimination and hate speech which are fuelling racist and, in particular, anti-Muslim feeling in the country," he said.