Story highlights The United States and United Nations condemn the bombings

The oil-rich city of Kirkuk has been the scene of tensions recently

Last month, a series of bombings there killed dozens and left more than 100 wounded

At least 20 people were killed and more than 35 others wounded when a suicide car bomber followed by gunmen in police uniforms attacked the police headquarters in the northern Iraqi city of Kirkuk on Sunday, city police said.

The Interior Ministry in Baghdad placed the casualty count higher: 30 dead and 70 wounded.

In the immediate chaotic aftermath of the attack, exact figures were difficult to come by.

Oil-rich Kirkuk, about 240 kilometers (150 miles) north of Baghdad, has been the scene of tensions recently. It is also one of the disputed areas in the country between the central government and Kurdistan regional government.

The United States and the U.N. Assistance Mission in Iraq condemned Sunday's attack. Martin Kobler, the U.N. special representative for Iraq, said the bombers "show a complete disregard for human life."

"It is important that the people of Kirkuk remain united against the forces of extremism and hatred," Kobler said in a statement issued by UNAMI. "I urge the Iraqi leadership to take all necessary steps that are required to deal with this alarming situation, particularly in Kirkuk, the symbol of peaceful coexistence in Iraq."

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A series of bombings in January killed dozens and left more than 100 wounded in Kirkuk. The attacks seemed targeted at Kurdish government officials.

The Iraqi central government and the Kurdistan regional government have been at odds for years over who has final authority over Iraq's vast oil wealth.

At the end of last year, relations between Prime Minister Nuri al-Malik's government and the Iraqi Kurds seemed to deteriorate further.

In November, Iraqi Kurdistan deployed troops and tanks to cement the Kurds' claim over the strategic city. This launched a tense standoff with units of the Iraqi army, which had been deployed to areas near Kirkuk.