— Insurgents inspired by al-Qaeda rapidly pressed toward Baghdad on Wednesday, confronting little resistance from Iraq’s collapsing security forces and expanding an arc of control that now includes a wide swath of the country.

By nightfall, the militants had reached the flash-point city of Samarra, just 70 miles outside Baghdad, after having first seized Tikrit, Saddam Hussein’s home town, and other cities while pressing southward from Mosul.

The stunning speed with which the rout has unfolded in northern Iraq has raised deep doubts about the capacity of U.S.-trained Iraqi security forces, and it has also kindled fears about the government’s grip on the capital.

In a country already fraught with sectarian tension, with parts of western Iraq already in Sunni militant hands, the latest gains by insurgents from the Islamic State of Iraq and Syria prompted cries of alarm from leaders of Iraq’s Shiite Muslim majority.

It appeared that the militants were facing more robust resistance as they moved south, where Iraq’s Shiites have a stronger presence. But several experts said it would be wrong to assume that heavily fortified Baghdad, with its large Shiite population and concentration of elite forces, could easily fend off an ISIS attack.

On Thursday, the militant group vowed to march on to Baghdad . A spokesman for the Islamic State of Iran and the Levant says the group has old scores to settle with the Shiite-led government in Baghdad, the Associated Press reported.

The spokesman, Abu Mohammed al-Adnani, also threatened that ISIL fighters will take the southern Iraqi Shiite cities of Karbala and Najaf, which hold two of the holiest shrines for Shiite Muslims. The statement, which could not be independently verified, came in an audio posting Thursday on militant Web sites commonly used by the group, the AP said.

Baghdad is “definitely vulnerable,” said Raoul Alcala, a former U.S. adviser to Iraq’s national security council who has spent most of the past decade in Iraq. “There are more troops in Baghdad, but it doesn’t mean it isn’t porous.’’

A separate analysis posted on the Web site of the Washington-based Institute for the Study of War said insurgents advancing from the north could link up with counterparts on the city’s perimeter to pose a real threat to the capital.

The Iraqi prime minister, Nouri al-Maliki, insisted for a second straight day that security forces were capable of reversing the militants’ gains. In a televised address to the nation, he pledged that Mosul, the largest city in northern Iraq, which fell to insurgents early Tuesday, would soon be back in government hands.

“This is just the latest round of fighting against ISIS, and it won’t be the last,” he said.

In Washington, the State Department said the United States is “expediting” the delivery of critical weaponry to the Maliki government but gave few details. “You can expect that we will provide additional assistance to the Iraqi government to combat the threat,” said Jen Psaki, the State Department spokeswoman.

ISIS on the march

Among those caught up in the fighting were dozens of Turkish citizens, including some diplomats, who were detained by militants during attacks in Mosul that included a strike on the Turkish Consulate there.

The conflict in the city, which began Monday evening, has sent hundreds of thousands of civilians fleeing, many to the safety of the semiautonomous Kurdistan region of northern Iraq. Civilians fleeing Mosul gave insight into why ISIS has been able to gain such a strong foothold in the Sunni-majority city, where anti-government sentiment is high.

Katheer Saeed, a 48-year-old truck driver, had left Mosul with his five children. He said he had been “excited” as he heard that the army had put down arms in the face of the ISIS advance.

He said he was fleeing because he feared a government air offensive rather than ISIS.

In western Iraq, the army has been accused of indiscriminate shelling and even using barrel bombs in its attempts to wrest back control of the city of Fallujah since it fell to insurgents in January.

Abu Mohammed, 50, agreed, saying he had left Mosul only because his father was sick. “ISIS just want to free the country from the unfair, sectarian government,” he said.

If the fighting reaches Baghdad, it is hard to see how a full-scale sectarian war can be avoided.

Among the worrisome signs to emerge Wednesday was a call by Shiite cleric Moqtada al-Sadr, leader of the now largely inactive Mahdi Army militia, to create a new security force to protect Shiite holy sites. Sadr accused the government of standing on the sidelines “shocked and silent” as the country fell between the “jaws of terrorism and extremism.”

A Western resident of Baghdad said fears were rising that the capital also could be vulnerable, and some foreign companies evacuated their personnel as a precaution Wednesday.

Iraqis also said they were frightened. “I want to take my family and leave, but I don’t know where to go,” said Nadeem Majeed, a Baghdad resident and father of two. “The road to Kurdistan isn’t safe, and Syria isn’t safe.”

Even in Sunni-dominated Mosul, it remained unclear how the militants had managed to achieve such a swift victory over government forces. By Wednesday, residents said, the city was festooned with banners declaring the creation of an Islamic emirate, while militants drove around the city once home to some 1.5 million people, announcing that life should continue as normal and that workers should return to their offices.

At a checkpoint seven miles outside Irbil, the capital of the Kurdistan region, a 33-year-old soldier who gave his name as Abu Sultan unzipped a bag to show his uniform, which he said he was not sure he would wear again, after nine years in the military.

He said that when his base south of Mosul came under fire Monday night, officers ordered the men to “leave everything and run away.”

“The leadership collapsed,” he said. “We left in our military vehicles, just with our AK-47s and handguns.”

Speaking at a news conference, Atheel al-Nujaifi, the governor of Nineveh province, of which Mosul is the capital, said army commanders had given assurances about their capabilities “only an hour before they got on a plane, leaving their weapons and fleeing.”

“This caused a total collapse of the security in the city of Mosul,” he said.

Oil companies said that so far the fighting had not affected Iraq’s 3.3 million barrels a day of oil production. To the south are the biggest oil fields — where Exxon Mobil, BP, the Chinese National Petroleum Corp. and others are working to boost production to pre-Hussein-era levels.

Other oil fields are located in the Kurdish-controlled area slightly east and north of the route the militants are taking toward Baghdad.

Despite the fighting, crude oil prices remained relatively flat on international markets. The price of Brent crude, the international benchmark, rose 43 cents, or 0.4 percent, to $109.95 a barrel.

“While the military is likely to redeploy part of its forces from the south, we do not anticipate a sharp deterioration in the security environment in these more stable provinces that would materially impact Iraq’s oil export volumes,” said Ayham Kamel, director for the Middle East and Africa at the Eurasia Group, a consulting firm.

A senior U.S. adviser on Iraq, Brett McGurk, was on the ground in Baghdad for emergency talks with senior government officials. Psaki, the State Department spokeswoman, described the situation as “very fluid’’ but said that Iraq’s largest oil refinery, in the northern Iraqi city of Baiji, remained in government hands.

Sly reported from Beirut. Anne Gearan and Steven Mufson in Washington contributed to this report.