For Air India, the country's only carrier with a major international network, fuel costs have risen by as much as 29% and journey times by up to three hours on some long-haul routes, according to the previously unreported document submitted to Indian officials in late October and reviewed by Reuters.

But after the Pakistan airspace closure, Air India's Delhi-Washington route was suspended in August. Now other routes are under review, with the direct Mumbai- and Bengaluru to San Francisco routes "becoming unviable" due to an additional three hours of travel time, including a technical stop in Kolkata, the document said.