DUBLIN/CHICAGO, Jan 28 (Reuters) - United Airlines has approached Airbus about buying more A321neo jets to fill a potential void left by the delayed Boeing 737 MAX 10, in a trade-off likely to ease deadlock over a long-delayed separate order for larger jets, industry sources said.

United CEO Scott Kirby flew to Toulouse recently to sound out the planemaker on a potential quid-quo-pro deal after a mid-air emergency on an Alaska Airlines 737 MAX 9 raised new doubts over certification of the already delayed MAX 10, they said.

"United Airlines has been in talks with Airbus about possible alternatives to the MAX 10 order. To my knowledge no agreement has been reached," a person familiar with the discussions said.

Airbus and United Airlines declined to comment.

Kirby's previously unreported trip to Toulouse is the latest twist in a widening crisis engulfing Boeing as the planemaker seeks to reassure the public and regulators about production quality and safety while preventing key orders unravelling. (Reporting by Tim Hepher, Rajesh Kumar Singh, Additional reporting by Valerie Insinna. Editing by Jane Merriman)