Updated

The U.S. ambassador to Iraq on Tuesday expressed concern about growing sectarian violence but said suddenly withdrawing U.S. troops would almost certainly make it worse.

Zalmay Khalilzad stopped short of calling the current strife in Iraq a civil war, arguing that state institutions are holding up and that state leaders say they want to stay in government.

"It's a matter of definition," Khalilzad said at the Center for Strategic and International Studies, a Washington-based think tank. "There is significant sectarian violence, no doubt about it."

He added: "Because of the desire of the leaders to work together and the state institutions to hold together, I do not believe the conditions I described are a civil war."

On Tuesday, Sunni Arab representatives said they will end their boycott of Iraq's parliament following a radical Shiite cleric's call for reconciliation. It came on a day of bombings and shootings that killed at least 39 people nationwide.

Noting that the process of nation-building in Iraq will take time, Khalilzad cautioned against a quick withdrawal of U.S. troops.

"A precipitous departure could unleash a sectarian civil war," he said.