BILLINGS, Mont. -- The cleanup of a major oil spill in the Yellowstone River has proved more difficult than expected and could go on for several more months, an Exxon Mobil Pipeline Co. executive said Thursday.

Areas hit hardest by the July spill should be cleaned up by the first half of October, said company vice president Geoff Craft. That includes a 20-mile stretch of the Yellowstone stretching from the spill site near Laurel downstream to Billings.

But scattered sites still would need to be dealt with, including contaminated river sections downstream of Billings and two large islands in the heavily affected area. Work in those areas could continue until Thanksgiving, Craft said.

Within days of the 1,000-barrel spill, Exxon Mobil was ordered by the Environmental Protection Agency to complete its remediation work by Sept 9. Officials have said since that the date was not intended to be a hard deadline.

Removing crude from hundreds of debris piles deposited by spring floodwaters has slowed the cleanup effort. Also, the company did not want to bring in more workers than necessary to avoid trampling the riverbank, Craft said.

"Nobody would have guessed how hard it would be," he said. "We don't want to do more harm than good by bringing in too many people or too many vehicles. ... It's very labor intensive."

Just more than 1,000 people are involved in the cleanup, with roughly 850 Exxon Mobil employees and contractors working along dozens of miles of riverbank.

Because the river was flooding when the pipeline failed, crude spread deep into the woods and into agricultural fields along some stretches of the river.

On Thursday, crews methodically picked their way through hundreds of acres of dense underbrush -- lopping off oil-stained plants and tree branches with hand clippers and then hauling the material away in plastic bags.

Nearby, a small excavator was pulling logs out of large debris piles that company representatives said would later have to be sorted by hand to remove anything stained with oil.

Despite the slow pace, state and federal regulators said significant progress had been made in the six weeks since the spill.

Teams sent out to find oil are no longer reporting many significant pockets of pooled crude that can be recovered, said Craig Myers, EPA's on-scene coordinator.

Instead, workers are concentrating on removing oil-stained vegetation and debris piles.

Remnants of the spill likely will linger long after the crews are gone, said Sandi Olsen, head of the remediation division of the Montana Department of Environmental Quality. But Olsen said any remaining splotches of oil are quickly degrading and are unlikely to pose a long-term threat.

"Our parameters for cleanup are that it does not pose a risk to human health for the environmental," she said. "A thin layer (of oil) -- that's going to be there until it weathers away. It's not going to pose a risk but you can see it."