BAGHDAD — A suicide bomber wearing a hidden belt of explosives attacked a police station in Iraq's volatile province of Diyala today, killing at least four policeman just days after a double suicide bombing in the same province left nine U.S. soldiers dead.

Today's explosion, which also injured at least 16 people, occurred at the front gate of the police station in a marketplace in Balad Ruz, 45 miles northeast of Baghdad, police said. All fatalities were policemen and the wounded included 11 civilians and five policemen, authorities said.

Since U.S. and Iraqi troops launched the security crackdown in Baghdad in February, Sunni militants are believed to have moved out of the Iraqi capital to seek haven in nearby areas such as Diyala.

Despite that, a U.N. report released today said that violence in Baghdad remains at high levels.

In its first human rights report since the security plan was launched on Feb. 14 — with increasing U.S. and Iraqi troops levels in the capital — the U.N. Assistance Mission for Iraq (UNAMI) said civilian casualties in the daily violence between January and March remained high, concentrated in and around Baghdad.

UNAMI also said that for the first time since it began issuing quarterly reports on the human rights situation in Iraq, the new Jan. 1 through March 31 report did not contain overall death figures from Iraq's Ministry of Health because it refused to release them.

The U.N. agency said the reason appeared to be that, after the publication of its last human rights report on Jan. 16, Prime Minister Nouri al-Maliki's office told UNAMI its mortality figures were exaggerated, "although they were in fact official figures compiled and provided by a government ministry."

"UNAMI emphasizes again the utmost need for the Iraqi government to operate in a transparent manner, and does not accept the government's suggestion that UNAMI used the (previous) mortality figures in an inappropriate fashion," the report said.