BAGHDAD » In an intensifying humanitarian crisis in Iraq’s embattled Anbar province, thousands of residents are fleeing a pitched battle between Islamic State militants and pro-government forces around the provincial capital.

Checkpoints on the main approach to Baghdad from western Anbar province are choked with cars, as the Iraqi authorities refuse entry to people who do not have a resident of Baghdad to vouch for them and provide them shelter. In normal times, a drive between the provincial capital, Ramadi, and Baghdad takes little more than an hour. Those lucky enough to reach Baghdad on Friday said they had been traveling for two days.

On the edge of Baghdad on Friday, just past a government checkpoint, Saad al-Thiabi, a police officer in Ramadi who has been battling Islamic State insurgents there for more than a year, was dropping off his family. He said he would return to the fight after catching a night’s sleep and doubted that he would ever see his family again.

"The main reason I’m taking them here is because I don’t want to be beheaded in front of my family members," he said, his young son standing by his side. "I want to be killed away from them."

The unfolding crisis in Anbar, coming just after the government promised a new military campaign to retake the province from the Islamic State, also known as ISIS or ISIL, suggests that the battle could be a monthslong, grinding affair.

Even while pressing its offensive in Anbar, the Islamic State demonstrated its ability to directly target Americans on Friday in a suicide car bombing near the U.S. Consulate in Irbil, the capital of the Kurdish region in northern Iraq.

The attack was deadly to civilians in the vicinity of the consulate but ultimately unsuccessful in its aim. Deeming the vehicle suspicious, security forces opened fire on it, and it exploded before it could reach the consulate, officials said. Still, the attempted attack was likely to raise fears about the vulnerability of U.S. diplomatic facilities in Iraq.

In Anbar, the government’s struggles also raise anew the question that has become a difficult one for U.S. military planners advising the Iraqi government and coordinating an intensified campaign of coalition airstrikes: What role, if any, should Shiite militias, backed by Iran, play in the fight for the Sunni heartland of Anbar?

The militias, which played an important role in the recent victory in Tikrit, Saddam Hussein’s hometown, have been absent, for the most part, from the fighting in Anbar. This is partly because of demands by U.S. officials, who worry that the presence of militias, despite their fighting prowess, could raise sectarian tensions in Sunni-dominated Anbar.

Now, as the situation has deteriorated, some Anbar officials and tribal sheikhs have called on the government to send militia units, which are grouped under an organization called the Popular Mobilization Forces.

The governor of Anbar, Suhaib al-Rawi, called on the Shiite religious authorities to support the people of Anbar and help "to stop the vicious offensive by ISIS." This was interpreted as a plea for help from the Shiite militias.

At Friday Prayer in Karbala, the Shiite holy city in southern Iraq, a representative for Grand Ayatollah Ali al-Sistani, the most important Shiite cleric, called on the people in areas under Islamic State control to liberate their own territories. But he also endorsed a role for the militias, saying, "It is all right for other Iraqis to participate with them to liberate their areas, despite their different affiliations or names, because at the end they are all sons of Iraq."

Mueen al-Kadhumi, a senior commander in the Badr Organization, one of the most prominent Shiite militias, said some of the same officials now asking for their help had just recently criticized the role of militias in the wake of the victory in Tikrit, where some units were accused of looting, burning homes and in some instances, extrajudicial killings.

"They were disparaging the popular mobilization and now they want it," al-Kadhumi said. "It is not a chess game."

Sabah Karhut, the chairman of the Anbar Provincial Council, has not called for the militias to be sent to Ramadi but said: "Ramadi today is surrounded on all sides by ISIS. We are in a critical situation."

Parts of Ramadi, but not the city center, have either been controlled or challenged by the Islamic State for nearly 16 months, well before the insurgents stormed in to Mosul, Iraq’s second-largest city, in June.

While fighting surrounded the city over that time, there were pockets within where life went on almost as normal, said residents.

"Two days ago, it completely changed," said Adil Raheem, who reached Baghdad on Friday. "We had to leave, otherwise we would die."

A man standing next to him called the soldiers and police defending Ramadi "cowards" and suggested that the city was at imminent risk of falling to the insurgents.

U.S. officials have warned that the city could fall and have played down the strategic significance of Ramadi to the efforts of the U.S.-led coalition against the Islamic State in Iraq and Syria. Iraqi government officials and security officers say the situation in the city is critical, with constant fighting, but are confident it can be defended.

The violence convulsing Ramadi presented a grim backdrop to the visit this week to Washington by Prime Minister Haider al-Abadi, who sought more U.S. assistance in the fight against the Islamic State and promoted the military campaign in Anbar.

Al-Abadi arrived back in Baghdad on Friday evening and headed directly to the city’s joint command center to be briefed on the situation in Ramadi. He then ordered that police and army reinforcements be sent to Ramadi, officials said.

The United Nations on Friday said that 4,250 families had fled fighting in Ramadi in recent days. Iraq’s sectarian divisions have made the situation worse: The authorities with the Shiite-led government in Baghdad are reluctant to ease the entry of large numbers of Sunnis from Anbar, and the United Nations said that at one checkpoint officers had orders not to allow any refugees into Baghdad.

Al-Thiabi, the police officer from Ramadi visiting Baghdad on Friday, said he had not been paid in two months but that he would go back anyway, even though he expected to die.

"I want to defend my house, my city, my province," he said. "These are not Muslims, they are criminals. This is not Islam."

Describing the scenes in the city in recent days, he said: "Bodies in the streets. Bodies burned in cars."

Tim Arango, New York Times