The leaders of North Korea and China have marked the 75th anniversary of their diplomatic relations by exchanging messages that expressed hopes for stronger ties, as outsiders raised questions about their relationship.

The alliance between China and North Korea — and more broadly, Iran and Russia — is a growing security concern for the West.

Leaders of China and North Korea on Sunday traded a vow of enhanced bilateral cooperation on the 75th anniversary of the establishment of diplomatic relations between the two countries, state news agency KCNA reported.