The U.N. human rights expert on Myanmar said on Tuesday that the ruling junta had murdered and tortured civilians while waging a "campaign of terror" in the year since taking power and he called for tightening sanctions against it.

Tom Andrews, a former U.S. congressman serving in the independent post, said that he had received more reports of "mass killings, attacks on hospitals and humanitarian targets and the bombing and burning of villages" in recent months.

A spokesman for the junta did not respond to calls seeking comment.

"The military junta is functioning as a criminal enterprise, committing murder, torture, abductions, forced displacement, all the while stealing the revenue and seizing the assets that rightfully belong to the people of Myanmar," Andrews said.

"They are then using these resources to buy the means to systematically attack the people of Myanmar. This must be stopped," he said in a video and statement marking the first anniversary of the junta ousting the elected civilian government led by Aung San Suu Kyi.

"The military is committing crimes against humanity and war crimes against the people of Myanmar," Andrews said.

Streets in some of Myanmar's main cities were nearly deserted on Tuesday as opponents of military rule called for a "silent strike" to mark the anniversary.

The United States, Britain and Canada imposed new sanctions on Myanmar's military after a year of chaos.

Andrews called again for the U.N. Security Council to impose a comprehensive arms embargo on Myanmar.

At least 1,500 people are known to have been killed in year-long protests against the coup, with thousands more possibly killed in the armed conflict, the U.N. human rights office said on Tuesday.

U.N. human rights spokesperson Ravina Shamdasani said at least 11,787 people were unlawfully detained in Myanmar in that period, including 8,792 who remain in custody.

Myanmar's ruling junta has disputed similar past estimates of the death toll and numbers in custody made by rights groups.

Shamdasani announced the figures for arbitrary detentions at a U.N. briefing in Geneva, adding: "This is for voicing their opposition to the military, whether in peaceful protests or through online activities even."