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Najim Rahim via The Associated Press
In this picture taken on Tuesday, Sept. 29, 2015 Afghanistan's security forces patrol of Kunduz city, north of Kabul, Afghanistan. The U.S. military says it has conducted two more airstrikes overnight on Taliban positions around a northern Afghan city seized by the insurgents this week.

KABUL, Afghanistan — U.S. airstrikes hit Taliban positions overnight around a key northern city seized by insurgents this week as Afghan troops massed on the ground Wednesday ahead of what is likely to be a protracted battle to retake Kunduz.

But in a setback to Afghan troop efforts, the Taliban forced government troops to retreat from an ancient hilltop fortress overlooking Kunduz on Wednesday, giving the insurgents a vantage point over the city.

Overnight, there was fierce fighting for control of Kunduz's airport, a few kilometers (miles) outside the city, before the Taliban retreated under fire, several residents said. The airport remained in Afghan government hands.

Taliban fighters were also gearing for the long fight and were seen planting bombs and mining roads in and out of the city on Wednesday to slow down Afghan forces.

U.S. Army spokesman, Col. Brian Tribus, said there were two new airstrikes and that U.S. and NATO coalition advisers, including special forces were at the scene "in the Kunduz area, advising Afghan security forces."

A lawmaker from Kunduz, Malim Chari, told The Associated Press that the fortress of Bala Hissar fell early in the afternoon on Wednesday. The structure dates back to pre-Christian times when the region was part of the Achaemenid Empire, founded by Persian King Cyrus the Great.

The Afghan army had used the fortress as a security post.

The Taliban captured Kunduz, a city of 300,000 people, on Monday. It was the first major urban area they seized since the 2001 U.S.-led invasion ousted their extremist regime.

The attack took Afghan authorities by surprise, as the militants managed to sneak into the city during the recent Muslim holiday of Eid al-Adha, a busy season when many Afghans travel in and out of urban areas.

The infiltration was an apparent intelligence failure, and the head of the country's intelligence agency, Rahmatullah Nabil, apologized to lawmakers in parliament on Wednesday for it.

Since the capture, the Taliban have put Kunduz on lockdown. Militants have been going house to house searching for government workers, instilling fear, according to residents who spoke on condition of anonymity, fearing for their safety.

Roads in and out of the city were blocked and the Taliban — believed to have joined forces with other insurgent groups to boost their numbers — were said to be forcing boys and young men to fight with them.

During Monday's assault, the insurgents freed 600 prisoners from the Kunduz jail, among them 144 who had been jailed as Taliban gunmen, officials said.

The insurgents also set up checkpoints to ensure that no one leaves. Officials who made it to the airport on the outskirts of the city before roads were sealed were still hunkered down there. The whereabouts of the provincial governor, Omar Safi, who was abroad when the city fell, were unknown.

Information from inside the city remained sketchy. Kunduz residents have described an atmosphere of fear and reported arbitrary acts of violence, such as torching and looting of government buildings, shuttered businesses and the compounds of non-government organizations, including the U.N. The road blocks were preventing delivery of food, medicines and other supplies into the city.

The U.N. special representative in Afghanistan, Nicholas Haysom, said he was concerned about reports "of extrajudicial executions, including of health care workers, abductions, denial of medical care and restrictions on movement out of the city."

Reports from the region indicated that up to 6,000 civilians have fled the city to escape the fighting, a statement from Haysom's office said.

The spokesman for Afghanistan's Public Health Ministry, Wahidullah Mayar, said on his official Twitter account that 30 people have been killed in the fighting, and more than 200 inured. "Around 90 percent of them are civilians," he tweeted.

The U.N. Assistance Mission in Afghanistan called on all parties in the conflict to "protect civilians from harm and to respect human rights at all times."

Kunduz, 175 kilometers (108 miles) north of Kabul, has been the scene of Taliban attacks since April, when the insurgents launched their annual warm weather offensive with an attempt to take control of the city.

The surrounding province, also called Kunduz, is one of the country's most important grain producers, has rich mineral resources and borders Tajikistan. Its strategic routes to neighboring countries in the east, north and west and to the capital Kabul to the south mean huge sums can be earned from smuggling of drugs, minerals and weapons across borders.

Associated Press writers Rahim Faiez and Amir Shah in Kabul, Afghanistan, contributed to this report.