Thousands of families fleeing Iraq's western city of Ramadi choked checkpoints leading to Baghdad on Friday, after an Islamic State advance spread panic and left security forces clinging to control.

A column of traffic several vehicles wide snaked for miles at a checkpoint in Sadr al-Yusufiyah, on the edge of Baghdad province, as minibuses, cars and trucks picked up families who crossed by foot carrying their possessions in bags and wheelbarrows. Suhaib al-Rawi, the governor of Anbar province, of which Ramadi is the capital, described it as a human disaster on a scale the city has never witnessed before.

U.S. and Iraqi officials have warned that the city is at risk of falling to the Islamic State despite seven months of airstrikes by American planes in Anbar. Such a loss would be a serious blow to Iraq's government, which recently announced a military campaign for the province after retaking the militant stronghold of Tikrit, and to the international effort to push back the militant group, whose gains in Ramadi have demonstrated its ability to create chaos even while under pressure.

That resilience was further underscored in the Kurdish city of Irbil on Friday, where the Islamic State was suspected of carrying out a car bombing near the U.S. Consulate. Faced with the expanding crisis on his return Friday from Washington, Prime Minister Haider al-Abadi ordered immediate reinforcements to Ramadi amid claims that some Iraqi security forces had withdrawn.

"The situation is critical right now," Rawi said of the teetering security in Ramadi. "Such a large wave of displacement has never happened in the history of the city."

Raad al-Dahlaki, head of the Iraqi parliament's committee for the internally displaced, said that 10,000 people had crossed into Baghdad province but that about 20,000 more remained stranded at the checkpoint because of a guarantor system, which requires fleeing families to have someone vouch for them.

Many arriving Friday in Sadr al-Yusufiyah had spent days traveling and said that few civilians had remained behind. Some were police officers who said they had left their positions after other security forces retreated and their ammunition had run low.

"How can you fight with only 20 bullets?" said a 47-year-old police officer, who said he had left the Malab neighborhood of Ramadi three days earlier with 18 members of his extended family. Like others interviewed, he did not want his name published because he had abandoned his post. He said the army had also withdrawn, a claim that could not be immediately verified.

"Daesh has M-16s and M-4s, and we only have Kalashnikovs," said another police officer, using the Arabic acronym for the Islamic State. He said he had fled his home in Ramadi's Soufiyah neighborhood with his wife and 1-year-old child. "I don't think I will ever see my house again."

Rawi said that there had been "realignments" of forces but not retreats and that there were assurances from the U.S.-led coalition that airstrikes would increase. Still, he said, support has been sorely lacking.

"We don't know if it's neglect or just a lack of capacity," he said.

Brig. Gen. Tahseen Ibrahim, a spokesman for Iraq's Ministry of Defense, said reinforcements from counterterrorism units had been deployed.

"Our troops are preparing themselves to attack," he said. Discussions were underway as to whether to also send what are known as popular mobilization forces, which include Shiite militias, but there was not yet an agreement, he said.

The question of sending the largely Shiite paramilitary forces has been contentious in Anbar, a predominantly Sunni province. But as the security situation has deteriorated, a growing number of local tribal leaders and officials have said they need all the help they can get. In his sermon Friday, Iraq's top Shiite cleric, Grand Ayatollah Ali Sistani, said "all sons of Iraq" should help the fight, a comment viewed as an endorsement of the militias playing a role.

At a Pentagon briefing Thursday, Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, played down the importance of Ramadi, saying that it is "not symbolic in any way" and that Baiji, a key location for Iraq's oil infrastructure, is "a more strategic target." But Iraqi military officials have said that securing Anbar province, much of which is already controlled by the Islamic State, is an essential step before any advance on Mosul, the group's base of power in Iraq.

That view was echoed Friday by Sen. John McCain, R-Ariz., chairman of the Senate Armed Services Committee, who called Dempsey's remarks a "gross mischaracterization."

"The fall of Ramadi would be seen by Iraqi Sunnis as a failure of the Baghdad government to protect them, and could deal a major blow to political reconciliation efforts that are essential to defeating ISIL," McCain, using another acronym for the Islamic State, said in a statement Friday, which was released jointly by Sen. Lindsey O. Graham, R-S.C. "Yet apparently, the current U.S. strategy is to defend an oil refinery in Beiji, but abandon the capital of pivotal Anbar province to ISIL."

Even if Iraqi security forces are able to shift the momentum to their favor, the setbacks in Ramadi have compounded Iraq's humanitarian misery, with the United Nations reporting that at least 4,250 families have fled the city in recent days.

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Saad Hamid, 40, said he left as Islamic State militants began to enter his neighborhood this week.

"We left on foot through the farms," he said. "They were terrified," he added, gesturing to his three young nieces and nephews.

Before reaching Sadr al-Yusufiyah, fleeing families were forced to leave their cars in Anbar province before crossing a bridge into Baghdad province on foot — a security measure authorities said was intended to prevent car bombs.

Dahlaki said that the security measures were necessary but that his committee has requested the guarantor system be dropped to ease the movement of families seeking to leave.

Sunni tribal sheiks who remain in Ramadi say they have received little support for their fighters, despite promises of arms from the central government. The city has been fending off Islamic State attacks for more than a year, after being overrun for the first time in January 2014. At that time, before the Islamic State had built its brutal reputation, fewer people fled. Now, things are different.

"People are practically dying of fear," said Omar al-Alwani, a tribal sheik who says he has about 500 fighters in Ramadi. "Half the army has retreated. It's only really the counterterrorism units now."

He said he was considering retreating with his men on the one remaining open road out of the city.

Tahseen, the Defense Ministry spokesman, disputed the sheiks' accounts.

"Some sheiks say the government is still weak about help, but it isn't true," he said.

Other fleeing Anbar residents said their local leaders were as much to blame as the central government for their plight, with many fleeing the province themselves.

"We were sold out by our sheiks," Hamid said. "They never found a solution for us, they just left. No one thought about the families."