NATO special forces have joined Afghan forces in their attempt to retake the city of Kunduz which fell into Taliban control on Monday.

The heavily assailed airport, which sits on a hilltop a few miles outside Kunduz, is now the only place held by the Afghan army. The nearby Bala Hisar fort fell when soldiers there ran out of ammunition, deputy provincial governor Hamdullah Daneshi said.  Thousands of troops have fled to the airport during the intense fighting over the last two days.

U.S. special forces had been advising Afghan troops while operating from a temporary base at the Kunduz airport for several weeks, according to a special forces commander.

Coalition spokesman Col. Brian Tribus gave few details about the foreign troops’ engagement with insurgents while supporting Afghan forces overnight, including the troops’ nationalities. Although the U.S. and NATO have officially handed over the battle against the Taliban to Afghan forces, the terms of their mission allows them to fight when they come under direct threat.

That happened early Wednesday morning when a team of U.S. special forces “encountered an insurgent threat in the vicinity of the Kunduz airport at approximately 1 am, 30 September,” Tribus told Reuters, adding that the soldier had acted in self-defense. “When they encountered the threat, they defended themselves,” he said.