IRBIL, Iraq -- The Islamic State of Iraq and Syria 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 ISIS, but that assertion was contradicted by hundreds of families who fled for safety to nearby areas controlled by Kurdish militias.
If confirmed, ISIS'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 also would end, at least for now, any claim by the central government in Baghdad to authority in northern Iraq and would allow ISIS 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 ISIS'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 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 also were 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 ISIS 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 ISIS'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 ISIS 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 ISIS-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."