Vistara, owned by the Tata group and Singapore Airlines, has cancelled more than 125 flights since April 1, one of the sources said, after a number of its pilots went on sick leave at the end of March.

Some of those who went on leave were protesting against a downwards revision to pay ahead of a merger with Tata-owned Air India later this year, amid growing fatigue due to a busy schedule, according to two Vistara pilots.

However, Vistara CEO Vinod Kannan said the shortage was mainly triggered by many pilots having reached their flight duty time limit, and not just because of the high amount of sick leave, the two sources said.

Kannan said the airline will look to take pressure off the pilots and from May onwards there would be "visible results" in rostering. Vistara will also look into scaling back its network but Kannan said that would need to be worked out, one of the sources added.

Vistara, which operates more than 300 flights a day, did not immediately respond to an email seeking comment out of business hours.

In a statement earlier this week, the airline blamed the cancellations on various reasons including availability of crew. It has deployed its Boeing widebody jets on some domestic routes to minimise disruptions.

The cancellations and delays forced India's civil aviation ministry to intervene and monitor the situation, while the aviation regulator has sought daily information on flight disruptions.

(Reporting by Aditi Shah; Editing by Kirsten Donovan)

By Aditi Shah