This image posted on a militant website on Saturday, June 14, 2014, which has been verified and is consistent with other AP reporting, appears to show militants from the al-Qaida-inspired Islamic State of Iraq and the Levant (ISIL) with captured Iraqi soldiers wearing plain clothes after taking over a base in Tikrit, Iraq. The Islamic militant group that seized much of northern Iraq has posted photos that appear to show its fighters shooting dead dozens of captured Iraqi soldiers in a province north of the capital Baghdad. Iraq's top military spokesman Lt. Gen. Qassim al-Moussawi confirmed the photos authenticity on Sunday and said he was aware of cases of mass murder of Iraqi soldiers.

IRBIL, Iraq — The Islamic State of Iraq and the Levant claimed control Monday of the last major city that had been held by the government in northern Iraq.

The government denied that Tal Afar had fallen to ISIL, but that assertion was contradicted by hundreds of families who fled for safety to nearby areas controlled by Kurdish militias.

If confirmed, ISIL’s capture of Tal Afar would allow the group to consolidate its control of a strategic supply corridor between its Syrian and Iraq strongholds.

It would also end, at least for now, any claim by the central government in Baghdad to authority in northern Iraq and would allow ISIL to claim for its nascent Islamist caliphate a contiguous territory that stretches from the Syrian city of Raqqa through Iraq’s Nineveh province to the outskirts of Baghdad.

The fall of Tal Afar would be freighted with historic import. American troops battled ISIL’s early incarnation, al-Qaida in Iraq, for control of the city in 2005. At one time, the pacification of Tal Afar was considered a major triumph for U.S. forces.

There were other signs of the collapse of Iraqi security.

In a letter, President Barack Obama told leaders of Congress on Monday that as many as 275 troops had been sent to Baghdad to augment security at the U.S. Embassy. The Pentagon had announced Sunday that additional troops would be sent, but did not provide a number.

There were also reports that a major battle was unfolding for the former U.S. military base at Taji, 20 miles north of Baghdad. Residents reached by phone reported that ISIL and its Sunni tribal allies had mounted a two-pronged assault on the base — from the west and the north.

The residents described heavy outgoing and incoming artillery fire and said Iraqi army forces were pulling back from checkpoints on the outskirts of town toward the base as they were attacked.

The Taji base, which was known as Camp Cooke to thousands of American service members in Iraq, is the country’s largest military facility, and the town is the most important population center still in government hands on the northern approaches to the capital.

Refugees and residents from Tal Afar who spoke with relatives in Irbil described Tal Afar as mostly in ISIL’s control, though some skirmishing was still taking place.

Kurdish officials speaking on local television said hundreds of Shiite Muslims had arrived at a checkpoint manned by Kurdish militiamen in northern Iraq. Video of their arrival showed hundreds of people queuing at a border post that links Nineveh province to the Kurdish autonomous region of northern and eastern Iraq, which has remained reasonably peaceful. Kurdish authorities have generally avoided being dragged into the inter-Arab sectarian civil war that’s raging through a large swath of western, northern and central Iraq.

Large numbers of Christians who fled villages outside Tal Afar also were among the refugees.

“They are driving the people from their homes,” said Avi, a Christian from Irbil who asked that his last name be withheld because he has family members living outside Tal Afar. “They murdered us in 2005, and the Americans could not protect us” he said, referring to al-Qaida in Iraq’s activities in the city during the earlier conflict. “Now the Iraqi government cannot protect us. There is nothing left in Iraq for Christians.”

The rapid advance by ISIL to the outskirts of Baghdad has put the capital on edge. On Monday, the United Nations announced that it would pull at least 58 staff members from its mission in Baghdad and move them to Amman, Jordan. The announcement came a day after the United States said it had reassigned an undisclosed number of staffers from its embassy in Baghdad to Amman or the relative safety of Irbil or Basra, in southern Iraq.

Iraqi state television said airstrikes targeted ISIS formations around the ISIL-controlled city of Tikrit, but the claims could not be verified.

Whether the United States would take any action to assist the Iraqi government remained uncertain. In comments Monday, Secretary of State John Kerry put special emphasis on U.S. frustration with Prime Minister Nouri al-Maliki.

“We are deeply committed to the integrity of Iraq as a country,” he said. “We are deeply committed to the constitutional process, but we’ve also had great difficulties with the existing government in their unwillingness to reach out and be inclusive and bring people to the table and be sufficiently responsible in their pluralistic approach to governance.”

Kerry appeared to lay the blame for the failure of the security forces, which the United States spent billions of dollars training before American troops left at the end of 2011, on favoritism by al-Maliki toward Shiites.

“That’s why you’ve really seen so many of these Sunni communities just melt away, because there is an ambivalence,” he said. “There’s a huge conflict in their own minds in their dislike of the existing government, but there are also terror and fear at the hands of a terrorist group.”

Kerry also appeared the leave open the possibility that the U.S. might coordinate any military effort — Obama has ruled out sending conventional ground troops but has said he asked the Pentagon to draw up other options — with Iran, which has already begun supporting the Iraqi government with advisers.

But Pentagon spokesman Navy Rear Adm. John Kirby discounted any cooperation with Iran, leaving the impression the Pentagon and the State Department were at cross purposes in deciding how to respond to the Iraq crisis.

“There is absolutely no intention to coordinate military activities between the United States and Iran,” Kirby said.

Asked whether Kerry had gotten ahead of the Obama administration in opening the door to U.S.-Iranian cooperation, Kirby responded: “I’m not going to speak for the secretary of state.”

A State Department statement later said that the subject of Iraq had come up briefly “on the margins” of negotiations in Vienna over Iran’s nuclear program, but discounted the possibility that the United States and Iran would coordinate military actions.

“We are open to engaging the Iranians, just as we are engaging other regional plays on the threat posed by (ISIL) in Iraq,” the statement said. “These engagements will not include military coordination or strategic determinations about Iraq’s future over the heads of the Iraqi people.”

That was consistent with what Josh Earnest, White House deputy press secretary, said: “We’re not interested in any effort to coordinate military activity with Iran.”

In his notice to congressional leaders about sending additional troops to Baghdad, required under the War Powers Resolution, Obama said the troops were “equipped for combat” and had been assigned to protect “U.S. citizens and property, if necessary.”

Pentagon spokesman Kirby later said the 275 included about 100 people to provide airfield management and logistics support “if required.”'

“The safety of personnel serving in diplomatic missions abroad is among our highest priorities. The presence of these additional forces will help enable the State Department to continue their critical diplomatic mission and work with Iraqis on challenges they are facing,'' Kirby said.

Prothero is a McClatchy special correspondent.