By Monica Vargas

SANTIAGO, May 15 (Reuters) - Chile's Chaiten volcano groaned, rumbled and shuddered on Thursday, raising new concerns among authorities, as lightning bolts pierced the huge clouds of hot ash hovering ominously above its crater.

Chile's National Emergency Office, ONEMI, said heavy ash kept shooting from the volcano in southern Chile as it generated small tremors.

On the ground, heavy flooding hit the area around Chaiten as falling ash swelled rivers, overflowing their banks.

"There's been additional volcanic activity that we're really worried about," regional governor Sergio Galilea told reporters.

The Chaiten volcano, 760 miles (1,220 km) south of the capital Santiago, started erupting on May 2 for the first time in thousands of years, spewing ash, gas and molten rock into the air.

The government on Wednesday declared the town of Chaiten, only six miles (10 km) from the erupting volcano, off-limits for three months and reported that about 90 percent of the town had been flooded by the Blanco and Raya Rivers.

"The flooding has receded in terms of water. But there's a lot of material left, more mud than water," Galilea said.

Rains are normal during the southern hemispheric winter in Patagonia, but the deluge of volcanic ash has caused nearby rivers to breach their banks.

No deaths have resulted, but thousands of people have been evacuated within a 30-mile (48-km) radius, including the 4,500 residents of Chaiten.

The column of ash above the volcano, kept aloft by the pressure of constant eruptions, rose as high as 20 miles (32 km) early in the eruption but has since fallen back to about 4.5 miles (7 km).

"The decision to evacuate was very opportune, as was the decision to keep the zone clear for now," said chief government spokesman Francisco Vidal after a meeting with President Michelle Bachelet on Thursday.

Chile's chain of some 2,000 volcanoes -- 500 of them potentially active -- is world's second-largest after Indonesia's. (Additional reporting by Damian Wroclavsky; Writing by Lisa Yulkowski; Editing by Eric Walsh)

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