U.S.-based Delta Air Lines has bought nearly 4 percent of shares of China Eastern Airlines for 450 million U.S. dollars.

Delta's investment will help China Eastern promote mixed ownership and speed up its overseas expansion.

The investment is part of a series of cooperation agreements signed by the two airlines, which both belong to the SkyTeam Alliance.

The two airlines have signed codeshare agreements to operate 30 U.S. domestic flights, 43 Chinese domestic flights and seven trans-Pacific flights.