IRBIL, Iraq — Al-Qaida-inspired insurgents battled government forces Monday in several locations across Iraq, as the United Nations said the insurgents almost certainly had committed war crimes by carrying out “cold-blooded executions” in their drive for power.

The stunning offensive by fighters from the Islamic State of Iraq and the Levant, ISIL, poses “an existential challenge” to Iraq and threatens the stability of the region, Secretary of State John Kerry said Monday. He said President Obama is considering “every option” available, including airstrikes. In a highly unusual move, Iranian and U.S. diplomats on Monday discussed possible cooperation to help stop the insurgents’ advance.

On Monday evening, Obama notified Congress that he was dispatching up to 275 military personnel to Iraq to provide support and extra security at the U.S. Embassy in Baghdad.

The move came after a week of fighting that has driven half a million Iraqis from their homes, and fanned fears of a bloody new civil war just three years after the departure of U.S. troops.

“This is a severe and dangerous developing humanitarian emergency,” Nora Love, the Iraq country director of the International Rescue Committee, said Monday.

U.N. human rights chief Navi Pillay said in Geneva that the Sunni jihadists have carried out an “apparently systematic series of cold-blooded executions” near the northern city of Tikrit in recent days that “almost certainly amount to war crimes.”

She said that according to corroborated reports from various sources, hundreds of noncombatants had been executed, including police and soldiers who surrendered or were captured, according to a statement from her office.

In addition, she said that, according to information received by U.N. employees in Iraq, the imam of the Grand Mosque in Mosul was killed for refusing to pledge allegiance to ISIL. The United Nations has received reports that a dozen local prayer leaders were executed under similar circumstances in front of Mosul’s al-Israa Mosque, she said.

On Sunday, ISIL released gruesome photos online that appeared to show the mass execution of prisoners in the central Iraqi province of Saladin, north of Baghdad. The pictures caused outrage and raised fears that supporters of the Shiite-dominated government might take revenge on Sunnis, leading to a sectarian bloodbath.

The rebels have overrun a large swath of western and northern Iraq, which they are seeking to combine with areas they control in neighboring Syria. On Monday, an official in the semiautonomous Kurdish region in northern Iraq told journalists that the jihadists had seized two major airports, three airstrips and 30 military bases, including four that American forces once used.

At the briefing, Jabar Yawar Manda, the general secretary of the Ministry of the Peshmerga – or Kurdish military forces – used a laser pointer to draw broad circles over the center of a map of Iraq. The area is in the hands of ISIL and its allies, he said.

“All the airports are controlled by Daash,” he said, using the Arabic acronym for ISIL. “All the money is controlled by Daash. The weapons are all under their control, from the tanks to the AK-47s,” he said.

The insurgents have threatened to extend their fight to Baghdad.

In a letter of notification to lawmakers under the War Powers Resolution, Obama said the military contingent he’s sending to Iraq “is deploying for the purpose of protecting U.S. citizens and property, if necessary, and is equipped for combat.”

The formal move comes after the administration deployed more than 50 Marines over the weekend to help protect the embassy. The State Department said it had moved some of the more than 5,000 personnel in Iraq to safer locations there and to Jordan. The embassy remains open.

Local news media reported clashes Monday between Iraq’s Shiite-dominated government forces and the Sunni jihadists in several areas north, east and west of the capital, including along Iraq’s border with Syria. Reuters reported that ISIL fighters and allied Sunni tribesmen overran the town of Saqlawiyah west of Baghdad on Monday, seizing six Humvees and two tanks.

The Associated Press quoted Iraqi security officials as saying that an army helicopter was shot down during fighting near the western city of Fallujah, killing the two-man crew. Insurgents also ambushed a vehicle carrying off-duty soldiers to Samarra, a town about 80 miles northwest of Baghdad, killing six and wounding four others, according to the news agency.

Fighting also was reported in Romanah, a village near a main border crossing into Syria in Iraq’s western Anbar province.

An Interior Ministry spokesman said Monday that Iraqi security forces had killed 56 “terrorists” and wounded 21 in operations just outside Baghdad in the previous 24 hours, AP reported.

Tens of thousands of Iraqi soldiers have fled the insurgent onslaught in the past week, some stripping off uniforms and boots as they retreated.

ISIL’s biggest prize has been Mosul, the largest city in northern Iraq. One resident of the city, who gave his name as only Abu Zakariya, said in an interview that “a lot of people” from the area have joined the insurgents. He referred to them as “revolutionaries.”

At least three explosions believed to be airstrikes rattled the city overnight Sunday, Abu Zakariya said.

“There are people getting killed – these are revolutionaries and the Islamic State,” he said, referring to ISIL. “We hear about martyrs, but we can’t confirm.”

Kurdish officials at the briefing Monday said the discriminatory practices and violent crackdowns on Sunni protesters over the past year by Prime Minister Nouri al-Maliki’s administration were among the factors behind the jihadists’ current success.

ISIL forces on Sunday took control of Tal Afar, a key city 30 miles west of Mosul. By Monday, at least 3,000 residents who fled Tal Afar reached the desert town of Sinjar near the Syrian border, said Love, of the Washington-based International Rescue Committee.

With ISIL expanding its control through northern Iraq, many of those fleeing have become trapped with no clear escape route. “People are frightened and confused – some have walked up to four days to reach Dohuk to escape the violence,” Love said, referring to a Kurdish city north of Mosul.

In an interview with Yahoo News, Kerry said U.S. drone strikes and airstrikes are among the options to combat ISIL.

“When you have people murdering, assassinating in these mass massacres, you have to stop that. And you do what you need to do if you need to try to stop it from the air or otherwise,” he said.

But the Pentagon sharply played down the possibility of military cooperation with the Islamic republic.

Iranian and American diplomats meeting Monday at a previously scheduled session in Vienna discussed the possibility of cooperation on Iraq, a State Department official said.