SEOUL, South Korea (AP) -- The leaders of North Korea and China marked the 75th anniversary of their diplomatic relations on Sunday by exchanging messages that expressed hopes for stronger ties, as outsiders raised questions about their relationship.

The message exchange came as North Korea and Russia have been sharply expanding their cooperation while China apparently keeps its distance. Experts say that the level of exchanges and commemorative programs between North Korea and China in the coming months will provide a clue to the exact status of their ties.