In recent weeks, fighters from the Islamic State of Iraq and the Levant have spearheaded a lightning offensive across Iraq, plunging it into its deepest crisis since the last US troops left in 2011. (Agencies)
The al-Qaeda breakaway group now controls territory stretching from northern Syria as far as the outskirts of Baghdad in central Iraq.
The figures issued by the UN mission to Iraq put last month's civilian death toll at 1,531, with 886 security forces killed. UNAMI added that 2,287 Iraqis, including 1,763 civilians, were wounded.
The figures exclude deaths in embattled Anbar province, which is largely controlled by Sunni militants.
The second deadliest month this year was May, with 799 Iraqis killed, including 603 civilians. April's death toll was 750.
The latest casualty figures exceed even last year's peak. The UN reported that last July at least 1,057 Iraqis were killed and another 2,326 were wounded.
"The staggering number of civilian casualties in one month points to the urgent need for all to ensure that civilians are protected," the UN Special Representative in Iraq, Nickolay Mladenov, said in the statement.
Mladenov called on Iraqi political rivals to "work together to foil attempts to destroy the social fabric of Iraqi society."
Iraq's new parliament holds its inaugural session on Tuesday. The country's top Shiite cleric urged lawmakers last week to agree on a prime minister before meeting in hopes of averting months of wrangling that could further destabilize the country.
The Islamic State meanwhile announced the establishment of its own government, or caliphate, ruled by Islamic law. It proclaimed its leader, Abu Bakr al-Baghdadi, a highly ambitious Iraqi militant with a USD 10 million US bounty on his head, to be the caliph, and it demanded that Muslims around the world pledge allegiance to him.
Through brute force and meticulous planning, the Sunni extremist group, which said it was changing its name to simply the Islamic State, dropping the reference to Iraq and the Levant, has managed to effectively erase the Syria-Iraq border and lay the foundations of its proto-state.
In recent weeks, fighters from the Islamic State of Iraq and the Levant have spearheaded a lightning offensive across Iraq, plunging it into its deepest crisis since the last US troops left in 2011.