ATLANTA — Delta Air Lines says it's buying a 4.3% stake in Korean Air's largest shareholder to strengthen its joint venture with the Asian carrier.

Delta said Thursday it invested in Hanjin-KAL and plans to increase its equity stake to 10%. The investment needs regulatory approval.

Hanjin KAL owns 30% of Korean Air, according to FactSet.

Delta isn't saying how much it paid for the stock.

The two airlines launched a joint venture last year, selling seats on each other's flights and sharing the revenue. Delta CEO Ed Bastian says it's already one of his airline's most successful partnerships.

Both carriers belong to SkyTeam, one of three alliances among global airlines. Atlanta-based Delta is the second-biggest U.S. carrier by revenue, behind only American Airlines.