North Korea again indirectly criticized China on Tuesday for its claims over the domination of Koguryo, saying it was an independent ancient kingdom of Korea.

"Koguryo had firmly preserved its national independence in its external relations and shattered any attempt to violate that independence," the (North) Korean Central Broadcasting Station (KCBS) said.

"Koguryo was a stately sovereign state, not an ethnic minority or provincial government or a tributary of any state power," it said, without mentioning China or details of the recent history dispute between China and South Korea.

Koguryo was a kingdom that stretched from the upper Korean Peninsula into what is today Manchuria in China between 37 B.C. and 668 A.D.

Koreans have no doubt that the kingdom is part of their history, but some Chinese scholars have recently laid claim to the kingdoms, arguing that it was a regional kingdom subject to China. The Chinese government supported that view after rejecting South Korean protests.

This was the latest in a series of North Korean news media reports on Koguryo and its successor Balhae. The North recently increased the volume of such reports in an apparent protest against what it sees as China's attempt to distort early Korean history. However, it refrained from directly criticizing its staunch ally.

Among the reasons for contradicting China's claims, KCBS cited the Great Wall of China built during the Qing Dynasty (221 B.C-206 B.C). "This proves that Koguryo was an independent country that had posed a grave threat to China from the beginning," it claimed.

Ancient documents show that Koguryo was ruled by kings who called themselves "emperor," had an independent year name and self-reliant foreign policies, the North said.

On Saturday, historians of the two Koreas held a joint photo exhibition of Koguryo tomb murals at Mount Geumgang on the North's eastern coast to mark the registration of Koguryo relics as a UNESCO World Heritage in July.

They issued a joint statement in which they agreed to deepen research into Koguryo history and introduce it to the world, the (North) Korean Central News Agency reported.