BISMARCK, N.D. (AP) – Federal oilfield inspectors are hustling to clear a backlog of hundreds of uninspected well sites on public and tribal lands in North Dakota, amid the explosion of drilling activity.

The Bureau of Land Management, which oversees drilling on federal land, said agency regulators are being brought in from other states to supplement North Dakota staff, with a goal by year’s end to catch up with inspections on “high priority” sites.

Keeping pace with the booming oil production has been a challenge, said Loren Wickstrom, a BLM assistant field manager in Dickinson.

“We’re swamped,” he said.

Still, the agency is making progress on its oil inspections, with about half the 335 sites identified as high risk being cleared from the list since October, Wickstrom said. His office also is in charge of inspecting well sites in South Dakota, where about half of the 20 high priority sites remain unchecked.

“We should get 100 percent checked this year (in the Dakotas),” Wickstrom said. Harsh weather slowed inspections over the winter but “the big push is right now,” he said.

BLM records say 116 of 696 high-priority wells drilled from fiscal year 2009 to 2012 in North Dakota and one of two in South Dakota were not inspected during that period. But they have since been checked, Wickstrom said.

The uninspected wells include new ones and those that might have fallen into the high-risk category.

The bureau provided The Associated Press with the national records giving a snapshot of three years’ worth of inspections. The agency oversees 100,000 oil and gas wells on public lands, about 3,500 of which received the high-priority designation. Nationally, about 40 percent of high-priority wells haven’t been inspected, showing a department struggling to keep pace with America’s drilling boom over the past decade.

In all, 19 states have high-priority wells and 13 states, including North Dakota and South Dakota, had high-priority wells that were uninspected.

BLM deems a well site high priority after performing a risk assessment on the site, based on production of the well, its location to an environmentally sensitive area and whether the well’s operator has had past compliance problems, Wickstrom said.

The intense focus on high-priority wells at present has meant that the hundreds of less risky wells in western North Dakota and dozens of those in South Dakota have largely remained unchecked in recent months. The agency has a goal of checking all wells at least once every three years and is still on track to do that, Wickstrom said.

Wayde Schafer, a North Dakota spokesman for the Sierra Club, said any untended or infrequently monitored oil well is unacceptable.

“The government is still playing catch-up and a large number of these wells are not being inspected,” he said. “Development is still outpacing the personnel needed to oversee what’s going on out there. We need to slow down and do this at a more reasonable and orderly pace. That will be better for everybody.”

North Dakota is the No.2 oil producer behind Texas. Almost a third of the estimated 1 million barrels produced daily in North Dakota is being collected from beneath the Fort Berthold Indian Reservation where BLM is tasked with oversight. About one-quarter of the more than 10,000 oil wells operating in North Dakota are on federal or tribal land, Wickstrom said.

BLM has 10 inspectors in North Dakota, with three of those hired in the past month. Another seven inspectors from other states have been brought in to help catch up, he said.

“At every level of BLM leadership, we are very much aware of our obligation and the struggles meeting these inspections,” Wickstrom said.

Even with the workforce that has been boosted by temporary out-of-state help, the agency could use at least 10 more permanent inspectors to keep pace, he said.

Schafer said the monetary wherewithal appears to be available to fund additional inspectors, since North Dakota’s oil bonanza has been a big financial boon to the federal government.

BLM data show that the federal government collected more than $247 million in royalties from drilling on public lands North Dakota in fiscal 2013, up from $102 million in fiscal 2011.

“North Dakota has contributed exponentially to federal coffers and the government should be able to fund enough inspectors to do it right,” Schafer said. “We can’t afford not to do it right.”