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Violence in Iraq soared in 2013 to levels not seen in years, U.N. officials reported this week, stoking concerns that the country is descending into the kind of sectarian bloodshed that gripped the country before the U.S. troop surge.

The United Nations said 7,818 civilians were killed in 2013, a return to 2008 levels. The startling figure follows warnings from lawmakers and analysts that the violence threatens to undo hard-fought gains by the United States.

"The level of indiscriminate violence in Iraq is unacceptable," U.N. Special Representative Nickolay Mladenov said in a statement, calling on the Iraqi government to curb "this infernal circle."

U.S. troops withdrew from Iraq at the end of 2011. In the years immediately before and after that withdrawal, the level of violence remained relatively steady -- and by Iraq standards, relatively low. The troop surge and other factors were credited with curbing the bloodshed from its 2004-2007 highs.

But the 2013 death toll is roughly double what it had been in recent years.

A number of factors are at play, and some are urging the Obama administration to get more involved -- though there is little appetite to send U.S. troops back into the country.

The violence in Iraq began to surge in April, after the Shiite-led government staged a deadly crackdown on a Sunni protest camp.

Iraq's Al Qaeda branch has fed on Sunni discontent and on the civil war in neighboring Syria, in which mostly Sunni rebels fight a government whose base is a Shiite offshoot sect. It has targeted civilians, particularly in Shiite areas of Baghdad, with waves of coordinated car bombings and other deadly attacks.

Another deadly car bomb was reported north of Baghdad on Thursday, as Al Qaeda-aligned fighters took over posts in at least four cities and towns in western Anbar province.

"We obviously condemn in the strongest terms the terrorist attacks we've seen," State Department spokeswoman Marie Harf said. "There's no place for this kind of violence in Iraq, and we are very committed to continuing to work with them to fight this common enemy together."

The U.N. figures gave a total of 759 people killed in December alone, including 661 civilians and 98 members of the security forces. Another 1,345 were wounded, the statement said.

The number of civilians killed in 2013 is higher than the 2008 toll of 6,787 -- though more were wounded in 2008 than in 2013.

Another group, Iraq Body Count, put the number killed last year at nearly 9,500, slightly lower than their estimate of 10,000 in 2008.

Even in the last few weeks, tensions have flared between Sunnis and Shiites in the country. The Sunni speaker of parliament said that dozens of deputies have submitted their resignations, demanding the Iraqi military pull out of Sunni cities.

Since last December, the Sunnis have been protesting against what they perceive as discrimination at the hands of the country's Shiite-led government and against tough anti-terrorism measures they say target their sect.

That tension, though, has been exploited by Al Qaeda in Iraq, as has the Syrian civil war next door.

At a Capitol Hill hearing last month, Washington Institute for Near East Policy fellow Michael Knights said young jihadists drawn into the fighting in Syria are also being "played into Iraq."

He cited figures to demonstrate the terror group's "resurgence." He said in 2010, car bombs declined to roughly 10 per month. In 2013, there was an average of 71 per month.

At the same hearing, Rep. Ileana Ros-Lehtinen, R-Fla., warned that the "resurgence" could mark the beginning of a bid to "create a large safe haven that spans from Syria to Iraq."

Knights urged the Obama administration to pay close attention to national elections in Iraq this coming April, calling this "job No. 1" -- since a corrupt election could give terrorists a "propaganda coup." More broadly, he urged for deeper engagement between the U.S. and Iraq to counter the perception that the U.S. has lost interest.

He and Kenneth Pollack, a senior fellow with The Brookings Institution, also urged the administration to step up counterterrorism cooperation, helping the Iraqi government identify terror targets and selling vital military equipment to Iraq.

While violence in Iraq surges, U.S. officials continue to engage with Iraqi leaders. Plus the U.S. continues to provide aid and arms to the Iraqis. The Stockholm International Peace Research Institute, which tracks weapons deals around the world, reported that the U.S. exported $316 million worth of arms and military equipment to Iraq in 2012. The U.S. sent nearly $1.3 billion in foreign aid to Iraq in fiscal 2012, though the number is expected to drop considerably this year.

The U.S. also has thousands of troops in next-door Kuwait.

Asked about the violence in Iraq back in November, White House Press Secretary Jay Carney put the blame squarely on Al Qaeda -- as opposed to the incumbent government. He defended the decision to fully withdraw from Iraq, despite pleas by some to leave a residual force, as the "right decision."

But he pointed to security assistance as key to helping lift Iraq out of its current state. "Iraq has to resolve the challenges that face Iraq through intense political effort, and aided by the security forces -- the troops and police who have been trained by the United States and our allies in that effort -- and through the assistance that we provide them and other friends of Iraq provide them going forward," he said.

The Iraq government is indeed under pressure to reconcile with the Sunni minority.

Iraq's Shiite Prime Minister Nouri al-Maliki recently appealed to Sunni tribes to team up with security forces to fight the militants, expressing his government's readiness to listen to residents in the restive province of Anbar to address their needs.

"I call upon on all generous tribes in Anbar to adopt a courageous stance ... and other tribes in (the provinces of) Diyala, Salaheddin, Ninevah, Baghdad and other parts of Iraq to come together," al-Maliki said in his weekly televised speech.

"I say that all our brothers in Anbar are welcomed for discussion or negotiations," he said. "We want them to come here (in Baghdad) so that we can put our hands together for the sake of Anbar to protect it from foreign agendas and Takfiri ideology." Takfirism refers to hard-liners who consider other Muslims to be infidels, and Maliki's invocation of it appears to be an attempt to remind Sunnis in Anbar, who were among the first to turn on Al Qaeda in 2005-2007, that they too had been targeted by extremists.

On Tuesday, Maliki said that the Iraqi Army would hand over control of cities in Anbar province to the local police, a main demand of discontented Sunni politicians who see the army as a tool in the hand of Maliki to target his rivals and consolidate power.

But Al Jazeera reported that Maliki later reversed his decision to withdraw soldiers from Anbar cities.

Amid the unrest, Harf said Monday that U.S. officials "have been intensely engaged from both Baghdad and Washington with Iraqi leaders on all sides."

"We've been urging restraint, dialogue, and certainly for all sides to take steps to de-escalate and not to further escalate the situation," she said.

The Associated Press contributed to this report.