BAGHDAD (CNN) -- The UN is unable to determine how many Iraqi civilians have been killed so far this year because the Iraqi government won't share the information, a UN agency said in a Wednesday report. An Iraqi government official denied that the information was withheld to cover up the number of civilian deaths in the war-ravaged nation, and the prime minister's office said the UN report "lacks accuracy." Even without the numbers, the report delivers a grim message: Iraq is facing "immense security challenges in the face of growing violence and armed opposition to its authority and the rapidly worsening humanitarian crisis."

The quarterly human rights report authored by the UN Assistance Mission for Iraq is considered the most reliable tally of civilians killed in Iraq, but Wednesday's report did not include the numbers for January, February and March.

Though it is lacking statistics, the report says anecdotal evidence indicates that assassinations and "large-scale indiscriminate killings" have kept the civilian death toll high, especially in and around Baghdad.

Violence also abounds in Nineveh, Salaheddin and Diyala provinces, as well as in Babil, just south of the capital.

Iraqi Prime Minister Nuri al-Maliki's office slammed the report, saying in a terse statement that it has "major reservations" about the document.

The report "lacks accuracy in presenting data, credibility in a lot of its points and its presentation of the human rights situation in Iraq," the statement said.

Though the Iraqi government desires transparency and wants to cooperate with UNAMI, much of the information in the report came from "unreliable, unofficial and unknown sources," according to the statement.

"The publishing of such an unbalanced report during the circumstances the country is going through puts UNAMI's credibility on the line and escalates the humanitarian situation in Iraq instead of resolving it," the government said.

UNAMI has seen its reports lambasted or ignored by the Iraqi government in the past, most recently in January when the agency reported that 34,452 civilians were killed and 36,685 were wounded in 2006.

The government initially told The Associated Press and Reuters it wouldn't comment on the report, and al-Maliki's office later told CNN the report used "exaggerated" figures.

In its Wednesday report, the UN says the Iraqi government provided no "substantive explanation or justification" for its decision to withhold information from the Ministry of Health and the Medico-Legal Institute, the capital's main morgue.

An Iraqi official, however, insisted it was an apolitical decision and said the government is revamping how it collects and distributes the data.

UNAMI noted in its report that al-Maliki's office has expressed displeasure with its previous reports.

After its January 16 report, "the prime minister's office told UNAMI that the mortality figures contained in the report were exaggerated, although they were in fact official figures compiled and provided by a government ministry," Wednesday's report said.