A suicide bombing attack left at least 35 people dead and more than 100 wounded, including Iraqi government officials, at a Shiite mosque in the northwestern town of Tuz Khurmato.
Martin Kobler, U.N. special envoy to Iraq and head of the peace keeping mission there, said such attacks did little to ease internal tensions.
"Such attacks aim to heighten tensions in this particularly sensitive region and I call on all people of Iraq to resist vicious attempts to undermine their thrive for peace and stability," he said in a statement.
Skirmishes late last year between Kurdish and Iraqi forces sparked concerns over the country's internal stability barely a year after U.S. combat forces left the country.
Victoria Nuland, a spokeswoman for the U.S. State Department, said Washington condemned the Wednesday attack in the strongest terms.
"The United States stands with the Iraqi people and will continue to support the government of Iraq, both against terrorist groups and to promote a religiously tolerant and diverse country," she said in a statement.
Increasing violence in Iraq comes as the country prepares for elections in April. Opposition to Iraqi Prime Minister Nouri al-Maliki has added to growing tensions in the country.