Al-Qaeda-inspired insurgents battled government forces in several locations across Iraq on Monday, 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 Syria (ISIS) poses “an existential challenge” to Iraq and threatens the stability of the region, Secretary of State John F. Kerry said Monday. He said President Obama is considering “every option that is 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.

As sectarian violence in Iraq escalates dramatically, what is at stake for the U.S.? The Post's diplomatic correspondent Anne Gearan, senior national security correspondent Karen DeYoung, chief White House correspondent Scott Wilson, and The Fix's Chris Cillizza weigh in on ramifications in the Beltway and beyond. (Julie Percha/The Washington Post)

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 officers and soldiers who had surrendered or been 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 ISIS. 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, ISIS posted gruesome photos online that appeared to show the mass execution of prisoners in the central Iraqi province of Salahuddin, 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 Pesh Merga — 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 ISIS and its allies, he said.

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

How the Islamic State is carving out a new country

The insurgents have threatened to extend their fight to Baghdad.

Obama said Monday that up to 275 U.S. military personnel will be sent to Iraq. “This force is deploying for the purpose of protecting U.S. citizens and property, if necessary, and is equipped for combat,” he said in a letter of notification to lawmakers under the War Powers Resolution.

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

Obama, who returned from a trip to California on Monday afternoon, held an early-evening White House meeting with members of his National Security Council to discuss options on Iraq that he had asked them to develop last week. Attendees included Vice President Biden, Secretary of State John F. Kerry, Defense Secretary Chuck Hagel, national security adviser Susan E. Rice, Attorney General Eric H. Holder Jr. and top military officials.

ISIS expands control

Local news media reported clashes Monday between Iraq’s government forces and the jihadists in several areas north, east and west of the capital, including along Iraq’s border with Syria. Reuters reported that ISIS fighters and allied Sunni tribesmen overran the town of Saqlawiyah west of Baghdad, 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 their uniforms and boots as they retreated.

ISIS’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 ISIS. “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.

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

With ISIS 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 ISIS.

“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.

Kerry also spoke with Yahoo’s Katie Couric about the possibility of collaborating with the Iranian government to curb advances by ISIS in Iraq.

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.

Liz Sly in Baghdad and Karen DeYoung, Anne Gearan, William Branigin and Craig Whitlock in Washington contributed to this report.