United Airlines Holdings Inc is offering its pilots a contract with an incremental value in excess of $8 billion over four years, in possibly the richest labor deal for a mainline U.S. carrier if ratified by the union.

The move comes at a time when pilots in North America are pushing for better pay and working conditions during their talks with company managements over new employment contracts.

Delta Air Lines (NYSE:DAL)’ landmark pilots contract, which offers $7 billion in higher pay and benefits in March has put pressure on rival carriers to hand out similar deals ahead of a busy summer travel season.

United’s CEO Scott Kirby (NYSE:KEX) told Bloomberg News that the company has been negotiating weekly to reach a formal agreement with the union.

However, he did not give details on the value of the pay, benefits and quality-of-life improvements included in the proposal other than to say that it would top the recent deals agreed to by its two biggest rivals, as per the report.

“We have disagreed with the company on the costing of items and they, in our opinion, have inflated many. We still have significant quality of work life, sick leave, long term disability issues open,” a spokesperson for the union, affiliated with the Air Line Pilots Association, told Reuters.

Meanwhile, shares of United Airlines rose 2.6% in morning trade, benefiting from broader market gains.

United did not immediately respond to a Reuters request for comment.