Eight centuries after the ruler of the greatest land empire in history was born, descendants of the mighty Mongol Kublai Khan are a people divided between his homeland and the China he conquered, with both claiming him as their own.

Kublai Khan's birthday was 800 years ago Wednesday, when Mongolia will commemorate the anniversary in Ulan Bator. In China a statue of him has been set up at the site of his summer capital during the Yuan dynasty, founded in 1271.

Under Kublai -- a grandson of Genghis Khan, who first started the Mongols' epic expansions -- the realm reached its greatest extent, stretching from eastern Europe to the Korean peninsula, the largest contiguous land domain ever.

But the Yuan emperors ruled China for less than a century, and after they fell the roles were reversed, with the Chinese later establishing themselves over Mongolia.

Geopolitical earthquakes in the 20th century, such as the collapse of China's Qing dynasty and the rise of the Soviet Union, finally saw Mongolia break away as an independent country, only to quickly fall under the sway of Moscow.

"Kublai Khan, being a Mongol, would have had great difficulty establishing control over China, so he had to make himself a Chinese emperor and thus found the Yuan dynasty," John Man, author and authority on Mongol history, told AFP.

"It's one of the world's greatest historical ironies that modern China gets most of its borders, minus Mongolia, from a barbarian from the North, from Kublai Khan who was a Mongol, not a Chinese at all."

Nonetheless China proclaims itself as the world's oldest civilisation and has a tendency to co-opt successful invaders and declare them Chinese.

Modern Mongolia has a population of only three million, the vast majority ethnic Mongols. But almost twice as many -- nearly six million -- live in the People's Republic of China, where they are one of dozens of minorities.

Some divided nations have reunified, such as West and East Germany. But despite some Mongolian nationalists' fantasies, the country's geopolitical weakness and economic dependence on China make a single Mongol state impossible, says D. Shurkhuu of the Institute of International Affairs in Ulan Bator.

"This is a very sensitive issue in political terms, especially for politicians in Mongolia," he told AFP.

- 'Great nation' -

On both sides Mongols agree on the glory of their shared history -- first brought to the West in "The Travels of Marco Polo", which recounts the Italian voyager's encounters with Kublai.

"Genghis Khan is the ancestor of ethnic Mongols and Mongolians," said Baigali, a guide at a complex in China's Inner Mongolia region billed as the mausoleum of Kublai's grandfather.

Foreign historians reject the claim, though the site of Genghis' grave has never been identified and remains one of the world's great unsolved historical mysteries.

Baigali, who goes by a single name, said the two peoples are essentially the same, although those in China use traditional vertical Mongolian script, while Mongolians write in the horizontal Cyrillic alphabet inherited from the Soviet Union.

"And they think they are superior to us because they are pure Mongolian and we are Sinicised," she added, hinting at underlying tensions.

Hada, an ethnic Mongol dissident who spent almost 20 years behind bars in China before being freed last December, says his people have been marginalised by Communist authorities and "downgraded... to an 'ethnic minority'".

"It is an undeniable fact that they are the indigenous people of a great nation," he wrote of China's Mongols in an article published online this month by the US-based monitoring group Southern Mongolian Human Rights Information Center.

"It serves a political agenda of the Chinese to belittle the Mongolian nation, diminish national self-confidence and cause them to abandon any aspirations of self-determination," he continued.

Beijing denies it oppresses minority groups and counters it has delivered economic development and raised living standards.

Mongol herders in China sporadically demonstrate against their resource-rich pastures being infringed upon by developers and coalminers -- one named Tumur hanged himself in protest earlier this year -- drawing attention and support from activists in Mongolia.

"There are many Tumur in Inner Mongolia and many herders are trying to keep their land away from the Chinese government," campaigner Munkhbayar Chuluundorj said in Ulan Bator, holding a sign reading "Je Suis Tumur", referring to the "Je Suis Charlie" movement that followed Islamist shootings at French satirical magazine Charlie Hebdo.

The impossibility of political unification did not preclude cultural connection, he said, invoking Kublai's grandfather.

"Genghis Khan is the only way because all Mongolians abroad believe they are proud of Genghis Khan," he said. "They want to say they are descendants of Genghis Khan."