WASHINGTON (Reuters) - The U.S. ambassador to Iraq has described sectarian violence as the main threat to stability and, warning of the global risk from an Iraqi civil war, said the government must act to curb it within six months.

Zalmay Khalilzad, in a speech in Washington on Tuesday, also cautioned fellow Americans that a hasty withdrawal could plunge Iraq into civil war and insisted U.S. political leaders must not block offers of amnesty to insurgents who have killed American soldiers if hopes of national reconciliation are to succeed.

"A year ago, terrorism and the insurgency against the Coalition and the Iraqi security forces were the principal sources of instability," he said.

"Violent sectarianism is now the main challenge ... It is imperative for the new Iraqi government to make major progress in dealing with this challenge in the next six months."

Khalilzad cited as reason for Americans to be "strategically optimistic" about Iraq the increased political engagement of the Sunni Arab minority previously dominant under Saddam Hussein and what he called a significant weakening of al Qaeda in Iraq.

"At the same time, the terrorists have adapted to this success by exploiting Iraq's sectarian fault line," he said.

"The central goal," he said, of Shiite Prime Minister Nuri al-Maliki's reconciliation plan to bridge that divide was to bring Sunni insurgents into politics -- a process that would require former guerrillas to be given amnesty from prosecution.

"Some in the United States reacted negatively to the concept of granting amnesties," Khalilzad said of strong comments by U.S. senators that killers of Americans must be pursued.

"We will work with Iraqi leaders to find the right balance between reconciliation and accountability ... There will not be a double standard that grants amnesty to those who killed soldiers in the Coalition but not to those who killed Iraqis."

Addressing Americans, the ambassador cautioned against pressure in Washington to bring troops home, saying it risked sparking a Sunni-Shiite sectarian war across the Middle East's oilfields and an ethnic separatist war involving the Kurds.

"A precipitous Coalition departure could unleash a sectarian civil war, which inevitably would draw neighboring states into a regional conflagration that would disrupt oil supplies," he said.

"It could also result in al Qaeda taking over part of Iraq ... This would make the past challenge of al Qaeda in Afghanistan look like child's play."

The Afghan-born Khalilzad, whom some majority Shiite leaders accuse of favoring his fellow Sunni Muslims as he tries to mediate, also said Iraq should curb "excesses" in the process of de-Baathification. Many Sunnis feel they are unfairly barred from office for having worked under Saddam's Baath party regime.

Also important for quelling Sunni unrest, he said, would be making good on promises to review the constitution drawn up last year in a process largely boycotted by Sunni leaders.