NATO is maintaining its diplomatic presence in Kabul and helping to keep the city's airport running, a NATO official told Reuters on Sunday as Taliban insurgents entered the Afghan capital.

"NATO is constantly assessing developments in Afghanistan," the official said, adding the security of the alliance's personnel was paramount and NATO would continue to adjust as necessary.

The official did not respond to questions on whether NATO planned to hold a crisis meeting to discuss the situation in Afghanistan.

Taliban insurgents entered Kabul on Sunday and said they expected to take power within days.

After almost two decades, NATO this summer completed military operations in Afghanistan and withdrew most troops from the country.

The alliance still operates a diplomatic representation in Kabul. A NATO spokesperson on Friday declined to provide details on the representation, citing security concerns, in response to a Reuters request.

Headquartered in Brussels, NATO also serves as a forum to coordinate national measures in Afghanistan, such as the evacuation of citizens that was discussed by NATO ambassadors on Friday.