A decrease in insurgent attacks in Baghdad that followed a surge of U.S. forces to Iraq has held, but violence elsewhere raise questions about the sustainability of peace, a United Nations report released Saturday determined.

As security improved in Baghdad, violent attacks spread last year to other parts of the country, including to Diyala Province and Mosul, Al Qaeda's last urban stronghold.

"The Government of Iraq continued to face enormous challenges in its efforts to bring sectarian violence and other criminal activity under control against a backdrop of political instability," U.N. analysts wrote.

The report, which covers a period from July to last December, reaffirms sentiments echoed throughout the military that the appearance of relative peace could dissolve.

But Staffan de Mistura, the U.N. chief in Iraq, said there is reason for hope.

"This is a window of opportunity for Iraq," he said during a news conference in Baghdad.

De Mistura said so-called Awakening Councils, groups composed of former Sunni fighters who have accepted U.S. backing to switch allegiances and fight Al Qaeda in Iraq, also have played a role in turning the waves of violence from sweeping the country.

The U.S. military in Iraq did not respond to e-mails from The Associated Press seeking comment on the report.

Also credited for the reduction in violence was an influx of thousands of U.S. forces to Iraq as part of a "surge" strategy ushered by Gen. David

Petraeus, the commander of U.S. forces in Iraq. The key goal was to secure the capital, give Iraq's politicians breathing room to cut deals that would bring minority Sunni Arabs into the government and thereby weaken or end the insurgency.

But the U.N. report cautioned against hasty conclusions because "the extent to which the decrease in violence was sustainable remained unclear." And violent attacks have grown more frequent in recent weeks.

Twelve U.S. soldiers have been killed in the last week, five of them in a single suicide attack in central Baghdad. A week ago two massive bombs hit Baghdad's Karradah neighborhood, killing 68 people.