An Iraqi security official said a suicide bomber detonated explosives in the northern Baghdad district of al-Taji, killing at least 22 members of paramilitary force as they lined up to get paychecks. At least seven other Iraqi soldiers were injured in the attack.
Kobler, U.N. special envoy to Iraq and head of the U.N. mission there, called Iraqi leaders to task as violence escalates in the country.
"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," he said in a statement.
His comments followed coordinated attacks on the northern city, which left dozens of people dead in an assault on a police headquarters.
Kobler in January said he was alarmed by the escalation of violence after the offices of political parties in Kirkuk were attacked.
Dozens of people were killed in late January attacks on protesters in Fallujah. Demonstrators there chanted slogans calling on Iraqi Prime Minister Nouri al-Maliki to step down.
Violence in Iraq surged when U.S. forces left the country in December 2011 under the terms of a bilateral status of forces agreement.