* Operator Statoil says not clear when fields will restart
* Fields shut after gas leak at Snorre A platform overnight
* Fields jointly produce about 157,000 barrels per day
* Safety watchdog: leak was probably big, inquiry started
(Adds safety authority, background on production)
By Wojciech Moskwa
OSLO, Jan 11 (Reuters) - Norway's oil and gas producer Statoil (STL.OL) said its Snorre and Vigdis fields in the North Sea were shut on Tuesday after a gas leak, further denting global oil output after an outage of a key Alaskan pipeline.
The North Sea fields -- which jointly produce about 157,000 barrels per day (bpd) -- were shut after gas leaked into the air on the Snorre A platform during a well shutdown.
"We still don't know when we will restart the fields," a Statoil spokesman said. "We hope to get production back soon."
Norway's Petroleum Safety Authority (PSA) said Statoil had stopped the gas leak and "normalised" the safety situation around 6 a.m. (0500 GMT) after the leak started at 3.38 a.m.
"We assume its a rather big leak but can't confirm it so far," PSA spokeswoman Inger Anda said. "As always, a hydrocarbon leak in a production area is very serious. Statoil has started an internal investigation and we have also asked for a meeting."
The 800-mile (1,280-km) Trans Alaska Pipeline, which carries oil from Alaska's North Slope, was closed after a leak was discovered on Saturday, shutting in flows equivalent to 12 percent of U.S. crude output. [ID:nL3E7CA0T6]
U.S. oil prices climbed above $90 per barrel on Tuesday as the Alaskan pipeline remained shut, removing more than half a million barrels per day from U.S. markets. [ID:nL3E7CB0A4]
According to the Norwegian Petroleum Directorate (NPD), Snorre produces some 116,000 bpd of oil along with small amounts of natural gas and natural gas liquids.
Vigdis, whose oil is piped to the Snorre A platform, produces some 41,000 bpd of oil, the NPD said.
Norwegian broadcaster NRK said platform workers spent several hours in lifeboats after the gas leak alarm sounded.
Norway prides itself on being a reliable supplier of oil and gas but its fields have been dogged by outages in past months, forcing the country's energy champion Statoil to cut its production guidance. [ID:nLDE6A209Z]
North Sea oil output is seen rising by 4.8 percent next month to above 2 million barrels per day, but over the course of 2011 oil production offshore Norway is expected to decline yet again as oilfields mature. [ID:nLDE70A169] [ID:nLDE70610N] (Reporting by Wojciech Moskwa; editing by Anthony Barker)