The United Airlines Holdings Inc pilots union has unanimously voted to authorize a strike vote, the union chair said in a letter to pilots on Friday.

"You'll receive more communication regarding this impending strike vote in the coming days," the letter says. A spokesman for the Air Line Pilots Association (ALPA) said it has not yet set a date for the strike vote.

This action comes months after pilots at Delta Air Lines ratified a contract that includes over $7 billion in cumulative increases in pay and benefits over four years.

Industry officials say Delta's new contract has become a benchmark for contract negotiations in North America.

"We have offered our pilots an industry leading deal and we're actively working with ALPA to reach an agreement," a United Airlines spokesperson said, adding that United flights will continue to operate as planned as the negotiation process continues.

A move towards a strike vote is a bargaining tactic unions have been using in contract negotiations, and a strike mandate does not mean pilots will walk off the job.

United's contract came up for renewal in 2019 and negotiations have been underway for five years. Last year, its pilots overwhelmingly voted against a tentative contract, which the union said fell short of what members were seeking. Since then, United pilots have been protesting for a better deal.

(Reporting by Kanjyik Ghosh in Bengaluru; Additional reporting by Gokul Pisharody; Editing by William Mallard)