United Airlines Holdings Inc said on Tuesday it expects a non-cash impairment charge of US$90 million in the fourth quarter related to its Hong Kong routes, following anti-government protests in the city.

The Hong Kong protests escalated in June over a now-withdrawn extradition bill, but have since developed into a broader movement.

"Due to a decrease in demand for the Hong Kong market and the resulting decrease in unit revenue, the company determined that the value of its Hong Kong routes had been fully impaired," the U.S. carrier said.

Several airlines including India's SpiceJet Ltd, Malaysia's AirAsia Group Bhd, South Korea's JejuAir Co Ltd reduced the number of flights to the city in the fourth quarter as the protests grew increasingly violent, disrupting daily life.