KABUL, Afghanistan — The U.S. military conducted an airstrike early Tuesday on the northern Afghan city of Kunduz that was captured by the Taliban the previous day, according to The Associated Press.

U.S. Army Col. Brian Tribus, spokesman for the U.S. and NATO missions in Afghanistan, said the airstrike was undertaken "in order to eliminate a threat to the force," the AP reported.

In a stunning multipronged attack, hundreds of Taliban fighters Monday seized the strategic northern city of Kunduz, the first major urban area to fall to the insurgents since the U.S.-led invasion of 2001.

Interior Ministry spokesman Sediq Sediqqi told The Associated Press that Kunduz “has collapsed into the hands of the Taliban.”

The New York Times reported that government forces had fled to the airport located about 6.5 miles south of the center of the city. Sediqqi said government troops had expected an assault but were stunned by the size and complexity.

The United Nations evacuated all its staff from Kunduz on Monday morning as the assault unfolded, spokesman Dominic Medley said.

The loss of Kunduz was a major setback to Afghanistan’s U.S.-supported forces, who have been struggling to curb Taliban assaults since the U.S. and its NATO allies ended their ground combat mission at the end of 2014.

Within 12 hours of launching their offensive at 2 a.m., fighters hoisted the white Taliban flag over the main square and ripped down pictures of President Ashraf Ghani and other government leaders. In a statement released online, the Taliban urged residents in the occupied city to continue their lives as normal, and instructed their fighters not to loot or engage in other abuses.

“Mujahedeen are not thinking of harassing or deriding anyone but have intentions of respecting and bringing serenity to their lives,” the statement said. “Mujahedeen are not thinking about retribution but have come with a message of peace.”

Afghan security forces were conducting strikes on the city overnight, with a plan to launch a counterattack on Tuesday, officials in Kabul told reporters.

As of Monday night, coalition warplanes had not conducted any airstrikes in support of the government troops, said an official with the NATO-led Resolute Support mission. NATO advisers based in Mazar-e-Sharif, about 100 miles west of Kunduz, continue to assist the Afghan security forces, the official added.

With government resistance crumbling, Taliban fighters swarmed through the city, seizing government buildings including a major hospital and freeing hundreds of prisoners from the local jail.

“The government has practically changed hands,” said Saad Mukhtar, the province’s director of public health, who spoke by phone while in hiding.

The insurgents had attacked Kunduz and other district centers before, but this marked the first time they have been able to push deep into a provincial capital in the months they have been testing Afghan forces no longer aided by combat support from the NATO-led coalition.

Afghan analyst Faheem Dashty told AP that government forces were “caught by surprise “ in what appeared to be a major intelligence failure. “They were expecting a big attack but couldn’t defend the city,” he said.

After earlier saying government troops had blunted the attack, Sayed Sarwar Husaini, spokesman for the Kunduz police, later confirmed that insurgents took control of significant sections of the city by the afternoon. The attackers freed as many as 500 prisoners from a jail in the city, he said.

Mass Taliban attacks on Kunduz, as well as on the Musa Qala district center in Helmand province in the south during a bloody fighting season, have taken on broad political and strategic significance and provided the insurgents with a major propaganda boost.

Afghan and coalition officials repeatedly have cited the Taliban’s inability to hold strategic territory — such as urban centers — as evidence of the success of a decade and a half of foreign military intervention and of progress by the coalition-trained Afghan forces.

Security forces largely had managed to hold on to or quickly retake the main cities and government centers across the country, sometimes with the help of American airstrikes and behind-the-scenes intelligence, logistics and special operations support.

Gen. Murad Ali Murad, deputy chief of the army staff, said the attack included Taliban insurgents from throughout the north as well as large numbers of foreign fighters, probably including members of the Islamic Movement of Uzbekistan.

“Strategic areas, including the airport, are controlled by Afghan security forces,” he told AP. “Reinforcements have already arrived and attacks on insurgent positions will be launched soon.”

Kunduz airfield, reportedly one of the bastions of government control in the city, still hosts a contingent of American special operations troops that are among those involved in a counterterrorism mission as well as training and supporting Afghan special forces. In recent battles in Helmand, U.S. special operations forces were among those aiding Afghan troops by calling in airstrikes.

Kunduz, a city of about 300,000 in 2013, is about 150 miles north of Kabul, near the Tajikistan border and is a major transportation hub for the north of the country.

Kunduz province and its capital city have been the scene of some of the most significant challenges for Afghan security forces, since the NATO-led coalition reduced its role to one primarily of training and advising Afghan forces.

Taliban units struck Kunduz from several directions about 2 a.m., according to police and Taliban accounts. One police official estimated the number of militants to be in the hundreds. Residents said fighting continued for hours, including artillery and small-arms fire, as well as rocket strikes by Afghan air force helicopters.

Afghan officials initially sought to play down the extent of the Taliban push, saying by midday that fighting was restricted to the outskirts of town.

Staff at the city’s central hospital posted photos on social media of Taliban fighters at the health facility on the city’s southern edge Monday morning. The guerrillas withdrew after an hour without harming anyone or causing any damage, only to return to seize the hospital later, Mukhtar said.

At least four dead civilians and 47 with injuries had been brought to medical clinics by early afternoon, Mukhtar said.

This offensive should be a warning to Afghan leaders, said Mohammad Jafar Kohistani, a professor of law and political science at Kabul University.

“Now is the time for our security officials to go on high alert,” he said. “They must come up with a clear and direct strategy against the insurgents. Otherwise, there will be more bloodshed and only innocent Afghans will be the victims.”

The Associated Press contributed to this report.

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