IRBIL, Iraq — Nearly two years after the U.S. troop withdrawal, Iraq is in the midst of a deepening security crisis as an al-Qaida affiliate wages a relentless campaign of attacks, sending the death toll soaring to its highest level since 2008.

In the latest violence, nine car bombs tore through markets and police checkpoints in Baghdad on Sunday, killing scores of people.

The bloody campaign has virtually erased the security gains made in the past five years. More than 5,300 Iraqis have been killed this year.

Sunday’s attacks occurred just three days before Iraqi Prime Minister Nouri al-Maliki is scheduled to arrive in Washington for meetings at the White House and on Capitol Hill. At the top of his agenda is a request for more U.S. help in the fight against the al-Qaida affiliate, whose scope has grown to encompass neighboring Syria as well.

“We need to increase the depth and width of our cooperation, to be more agile and reflect the seriousness of the situation in Iraq,” Iraq’s ambassador to Washington, Lukman Faily, said in a telephone interview. “In our discussions, we will highlight the urgent need for the approval and quick delivery of military sales.”

At least 40 people died in Sunday’s attacks on mostly Shiite neighborhoods in Baghdad, according to an Interior Ministry official, who was not authorized to be quoted by name. In addition, a suicide bomber plowed his car into a group of soldiers in the northern city of Mosul, killing 14, according to a local police official.

More than 600 people have been killed in Iraq this month, after extraordinary bloodshed left 880 dead in September.

The surge in violence is largely, though not exclusively, due to attacks by the Islamic State of Iraq and Syria, or ISIS, an outgrowth of al-Qaida in Iraq. The group is trying to undermine the Iraqi state through a relentless campaign of car bombings, assaults on security forces and assassinations. ISIS has also carried out several jailbreaks, freeing militants whom American and Iraqi forces worked for years to capture.

Iraq once looked as though it could be stabilizing. After a horrific sectarian war engulfed the country in 2006 and 2007, the United States sent a surge of troops and began enlisting Sunni fighters to turn against al-Qaida. When the U.S. military withdrew at the end of 2011, militant groups were on their heels and monthly civilian death tolls were in the dozens rather than the hundreds.

That period of relative safety lasted into 2012, but it began to unravel as Syria’s anti-government protest movement developed into a civil war fought along sectarian lines.

Many radical Sunni fighters in Iraq and Syria have now united under the banner of ISIS. The militants consider the two nations to be different fronts in a single war with an ambitious goal.

“They want a caliphate,” said Jessica Lewis, the research director at the Institute for the Study of War, who has investigated the resurgence of al-Qaida in Iraq. She was referring to the Islamic political system that ruled most Muslims after the death of the prophet Muhammad.

One of the group’s most important victories came in September 2012, when militants freed about 100 inmates from a prison in Tikrit. The infusion of manpower helped the organization intensify operations this past spring, Lewis said.

ISIS launched an even more devastating attack in the notorious Abu Ghraib prison on July 21, freeing more than 500 convicts who could bolster the group’s capabilities further in the coming months.

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