By Dmitry Solovyov

ALMATY (Reuters) - Six people were convicted on Monday for taking part in deadly labor riots in Kazakhstan that were seen as a major challenge to the authority of President Nursultan Nazarbayev.

Another six people were cleared of rioting during the two days of violence that left at least 15 dead in December, court officials said.

Violence broke out when a seven-month strike by oil workers in the western town of Zhanaozen spun out of control during celebrations of the 20th anniversary of Kazakhstan's independence from the Soviet Union.

At least 14 people were killed in Zhanaozen when the oilmen demanding higher wages clashed with police who used live rounds.

The next day fighting spread to the nearby village of Shetpe, where a large group supporting the oilmen stopped a passenger train, set a locomotive on fire and clashed with police. One of the protesters died of gunshot wounds.

Human rights activist Yevgeny Zhovtis told Reuters many had expected much heavier sentences would be handed out - and the fact so many defendants had been released might have been a bid to calm tensions.

"It is certainly an attempt by the authorities to save face and also demonstrate their force, while trying at the same time not to escalate tension any further in that region," he said.

The Mangistau regional court, located in the Caspian Sea port of Aktau in western Kazakhstan, said in a statement four people were jailed for between four to seven years each for taking part in the violence in Shetpe.

A fifth defendant was given a suspended three-year sentence and a sixth was released under a nationwide amnesty for minor crimes.

The verdict in another trial of 37 people charged over the violence Zhanaozen is expected in the next few days.

Authorities in Kazakhstan, a Central Asian ex-Soviet republic of 16.7 million, are wary of new outbursts of labor unrest in single-industry towns like Zhanaozen.

(Additional reporting by Mariya Gordeyeva; Editing by Andrew Heavens)