ANCHORAGE — Royal Dutch Shell on Monday was moving its drill ship off a prospect in the Chukchi Sea, a day after drilling began 70 miles off the Alaska coast because sea ice was moving toward the vessel.

Shell Alaska spokesman Curtis Smith tells The Associated Press that drilling was stopped for safety reasons.

“As a precautionary measure and in accordance with our approved Chukchi Sea Ice Management Plan, Shell has made the decision to temporarily move off the Burger-A well to avoid potentially encroaching sea ice,” he said by email. “Once the ice moves on, the Noble Discoverer will re-connect to anchors and continue drilling.”

Shell officials on Sunday were monitoring ice measuring 30 miles long and 12 miles wide about 105 miles away from the drill ship, Smith said by phone.

“We’re using satellite images, we’re using radar images, we’re also using onsite reconnaissance to watch this ice so there are no surprises,” Smith said.

The ice varies in thickness, he said, but at its thickest is 25 meters, or about 82 feet. It was moving at 0.5 knots, or less than 1 mph.

The decision to halt drilling was made Sunday. At noon Monday, the drill ship was detaching from the last of eight massive anchors. Smith said he did not know how far away the ice was at that time.

The anchors will stay in place.

“Part of working in ice is having the ability to temporarily relocate,” Smith said. “You never want to stop operations when your crews and your equipment are working smoothly but this is what it means to work safely in the Arctic.”

Drilling may be delayed by two days or more, he said. The ice, he said, is dynamic. After it passes, changing winds could blow it back.

“We need as much margin once it moves by as we demand before we start drilling,” he said.

Drilling had begun at 4:30 a.m. Sunday. Royal Dutch Shell PLC was given permission last month to begin preliminary work on an exploratory well. The company’s oil spill response barge has not been certified but the company was authorized to drill pilot holes that do not descend into oil reservoirs.

Shell has spent upward of $4.5 billion for Arctic Ocean drilling but had been thwarted from drilling by environmental lawsuits, regulatory requirements and short open-water drilling seasons.

Shell Alaska vice president Pete Slaiby on Sunday called the beginning of drilling historic. He said it was the first time a drill bit had touched the sea floor in the U.S. Chukchi Sea in more than two decades.

Drilling is bitterly opposed by environmental groups that say oil companies have not demonstrated they can clean up a spill in ice-choked water. They say a spill of the magnitude of the Deepwater Horizon blowout in the Gulf of Mexico would be catastrophic in a region hammered by climate warming and home to endangered or threatened marine mammals such as bowhead whales, polar bear and walrus.