The cleanup of a major oil spill in the Yellowstone River has proven 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, company vice president Geoff Craft said. 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 impacted area. Work in those areas could continue until Thanksgiving, Craft said.

Slowing the cleanup effort has been the painstaking task of removing crude from hundreds of debris piles deposited by the same spring floodwaters that are widely believed to have triggered the 12-inch pipeline's failure. Also, the energy 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," Craft 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."

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. But officials said Thursday that date was not intended as a hard deadline.

EPA on-scene coordinator Craig Myers said the cleanup "is much more dictated by progress in the field instead of a date on the calendar." Myers added that final approval of the work done by Exxon Mobil would have to come from Montana officials.

About 1,000 people are involved in the effort to mop up the spill, including roughly 850 Exxon Mobil employees and contractors working along dozens of miles of riverbank.

Because the river was flooding when the pipeline failed, the spilled crude spread deep into the woods and across agricultural fields, making it difficult in some cases to find and remove.

On Thursday, crews could be seen methodically picking 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 apart a tangle of logs and branches — one of many debris piles that company representatives said would have to be sorted by hand to remove anything stained with oil.

Despite the slow pace, state and federal regulators said significant progress has been made in the seven 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 Myers. Instead, workers are concentrating on removing oil-stained vegetation and the 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 deposits of oil were quickly degrading and 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," Olsen 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."

So far, government officials have completed re-inspections on six river segments with oil contamination after Exxon Mobil said it had largely completed its cleanup work in those areas. That's out of 167 contaminated segments, according to the EPA.

Exxon Mobil representatives said they expected the number of sites ready for re-inspection to increase rapidly in coming days as they move beyond the worst-hit segments that were a priority in the initial cleanup.