BEIJING, Dec 6 (Reuters) - China has raised the flow of water from behind its massive Three Gorges Dam to ease a downstream drought that is the worst in half a century, the official Xinhua agency reported on Thursday.

Low rainfall along the upper parts of the Yangtze River, China's longest, meant levels on the middle stretches had retreated 1.5 metres below average, stranding at least 26 cargo ships over the past month.

River authorities issued a warning to shipping about low water levels, and ships had to be checked for weight and unload excess cargo before heading through the drought-stricken section, the report said.

Over 1,000 workers and 100 boats were also digging out silt along the river, which is usually lower over the autumn and winter but replenished by spring floods, Xinhua added.

The flow of water has now increased by more than 5 percent from Tuesday levels, according to the China Three Gorges Project Corporation (CTGPC), raising downstream water levels around half a metre. The extra flow will last at least until next Tuesday.

Large areas of south China are also suffering from serious drought, with water levels on two major rivers -- the Gan and the Xiang -- in rice-growing provinces dropping to historic lows, state media said in November.

China suffers a water shortages of nearly 40 billion cubic metres a year which its Water Minister has blamed largely on global warming, state media have reported, although severe pollution and rising consumption by both farmers and booming cities have compounded shortages. (Reporting by Emma Graham-Harrison; Editing by Alex Richardson)

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