Baghdad ? The influx of thousands of U.S. forces has driven down insurgent attacks in Baghdad, but violence elsewhere in Iraq raises questions about whether killings will continue to drop as American forces begin to leave, the United Nations said Saturday.

As security improved in Baghdad, violent attacks spread last year to other parts of the country, including Diyala Province and Mosul, al-Qaida’s last urban stronghold, according to the report from the United Nations Assistance Mission for Iraq.

“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,” the report, which examined the last six months of 2007, said.

“This is a window of opportunity for Iraq,” Staffan de Mistura, the U.N. chief in Iraq, said at a news conference in Baghdad.

The U.S. military has said a 60 percent reduction in attacks followed the influx of more than 20,000 additional troops, known as the surge. Military officials did not respond to requests for comment.