UPDATE 1-BP says Alaska oil to return after power snag

(adds outage details, more comment from BP, paragraphs 3-6)

ANCHORAGE, May 9 (Reuters) - British oil major BP Plc (BP.N) said up to 400,000 barrels per day of oil production was shut down by power outage in Alaska's Prudhoe Bay early Friday, but should begin to return later on Friday and Saturday.

The outage downed six Prudhoe Bay processing plants and the Northstar field. "We anticipate by end of today three of the Prudhoe processing plants plus the Northstar will be operational," said BP spokesman Steve Rinehart. The three other Prudhoe units were expected back on Saturday, he said.

"Production will ramp up in the coming days as we get the processing facilities operational."

A snowplow early on Friday ran into a compressed-air line that feeds into the Prudhoe Bay power facility, triggering the outages, he said. The plow had been needed because of severe weather conditions, including high winds and ground blizzards, he said.

While backup generators were used during the outage, they do not provide enough power to operate the processing units, Rinehart said.

Most of the regular power was back on by Friday afternoon, but the processing units could not be started as quickly, he said.

"It takes a while to bring processing facilities back into operation because there are careful procedures they have to follow," he said. (Reporting by Yereth Rosen; Editing by David Gregorio)