In a crackdown, Air India on Tuesday sacked ten agitating pilots, de-recognised their union and sealed its offices as around 160 of them failed to join duty, leading to cancellation of five international flights since midnight.

With Civil Aviation Minister Ajit Singh terming their agitation "illegal", airline sources said more severe action was to follow if the pilots did not get back to work by 1800 hours today.

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Flights on Delhi-Toronto, Delhi-Chicago, Mumbai-Newark and Mumbai-Hongkong via Delhi sectors were cancelled "due to unavailability of pilots," an Air India spokesperson said.

With about 160 pilots not having joined duty, the services of at least ten pilots, including office bearers of the Indian Pilots Guild (IPG), which is leading the agitation, were terminated, the sources said.

While the IPG has been de-recognised, the sources said its offices in Mumbai and Delhi were sealed.

A section of Air India pilots owing allegiance IPG are agitating over the rescheduling of Boeing 787 Dreamliner training and matters relating to their career progression.

The Civil Aviation Minister termed the strike as "illegal" and said the management of the national carrier would take appropriate action against those involved in the stir.

"They are reporting sick. They have not given any notice for any strike. So whatever the Air India management rules and regulations, action will be taken accordingly," Singh told reporters.

"The pilots have been told to return to work by today evening. And if they don't, the management will take stern action against them," the Air India spokesperson said.

The airline management has also started sending doctors to the houses of pilots who were reporting sick, the sources said.