OSLO, Aug 9 (Reuters) - A chunk of an Arctic glacier broke into the sea and triggered a huge wave that injured 18 people on a sightseeing boat, almost all of them British tourists, Norwegian officials said on Thursday.

Four people were seriously hurt in the accident by Hornbreen glacier on the Norwegian archipelago of Svalbard and were flown south to a hospital in Tromsoe on the mainland. The others were treated at a local hospital, mostly for minor injuries.

"The glacier calved (split off) and a big wave washed over the boat," Elisabeth Bjoerge Loevold, acting governor of Svalbard, told Reuters. "The boat rocked back and forth and passengers fell on the deck."

"We believe there was no ice from the glacier directly on to the deck but we don't have all the details," she said. Some Norwegian media reports had earlier said the boat was showered with ice from the glacier.

The small cruise vessel, the Alexey Maryshev, was carrying about 50 British tourists and a crew of 20 Russians.

Loevold said 17 of those injured in the accident late on Wednesday were believed to be British and the other a Russian crew member, who was among those seriously hurt.

Boats are meant to stay clear of glaciers around Svalbard, which is about 1,000 km (600 miles) from the North Pole, in case chunks fall off. But there are no fixed rules for how far is safe.

"The captain and the expedition leader will be questioned about it by the police," Loevold said.

Glaciers naturally break apart as they slide downhill but many are shrinking more quickly than usual because of global warming, blamed by almost all scientists on human emissions of greenhouse gases from fossil fuels.

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