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One of the main ethnic Karen rebel leaders has been sentenced to 17 years in prison on a charge of "unlawful association", reports from Burma say.

Mahn Nyein Maung, of the Karen National Union, was deported to Burma after a visa problem on a trip between Thailand and China, Burmese journalists said.

Burma has not commented on the reports.

Burma's government says the KNU is illegal, but has recently held exploratory peace talks with it and other armed ethnic minority groups.

Mahn Nyein Maung was reportedly travelling from Thailand to China for talks about the peace process.

On his return to Bangkok, the Thai authorities allegedly found out that he had an invalid visa, sending him back to China. Beijing then deported him to Burma.

Mahn Nyein Maung was initially charged only with having false documents, reports say.

But when the Burmese authorities found out who he was, they laid the much more serious charge of unlawful association.

The KNU told the BBC it was aware of the reports of Mahn Nyein Maung's imprisonment and would raise the issue with government representatives.

The KNU has been fighting for greater autonomy since the late 1940s.