(Reuters) - American Airlines Group Inc on Friday said it raised the value of its contract offer to pilots by more than $1 billion to match a more generous deal at United Airlines after American's union warned that ratification was in jeopardy.

The Texas-based carrier said the changes bring the total value of the four-year proposed contract to $9 billion. Its pilots are due to vote on the deal on Monday.

American said it would match the pay rates and retroactive pay in United's tentative agreement reached July 15 and provide extended sick leave and increased life insurance.

United's pilots will get cumulative 34.5% to 40.2% pay raises in that four-year agreement.

The Allied Pilots Association (APA), which represents American's pilots, warned on Sunday that ratification was in jeopardy. Dennis Tajer, a spokesman for the union, said earlier this week that United's pilots would earn at least 2% more than their 15,000 counterparts at American.

The union also said United's back pay, days off for junior pilots and sick time were superior.

A shortage of pilots and increased public support for unions have given union negotiators greater leverage in contract talks.

(Reporting by Priyamvada C in Bengaluru; Editing by Cynthia Osterman)