— Raleigh-Durham International Airport officials said Thursday that hikers and mountain bikers have been trespassing on airport property for years, and a controversial 18-mile fence along the border of the property is needed to keep them out.

"No trespassing" signs posted in the area haven't stopped anyone, RDU Chief Operating Officer Bill Sandifer told members of the airport authority board. Bikers and hikers have even built an elaborate web of trails that include manmade bridges and signage, he said, noting the work is causing environmental damage.

"We’re not a park. We’re not a recreational activity. We need to keep unlawful activity off the property. That’s why we’re doing a fence," Sandifer said.

But the property abuts Umstead State Park, and opponents say the fence will cut off access to the park along Old Reedy Creek Road.

"A fence is totally inappropriate. That land has been open for a long time," said Natalie Lew, a member of RDU Forest, which also opposed the airport's lease of some property for a rock quarry. "It just harms everybody."

Sandifer said the trespassers also present a liability concern to RDU, citing two incidents in the last two years where EMS had to be called to extricate two people who suffered severe injuries from the wooded area that would be fenced off under the proposal.

"We have no intention of cutting off access to Umstead Park," he said, noting he plans to meet with park officials on Jan. 9 to discuss the issue.

Umstead Park could lease some land from RDU, which would also change the route for the fence, he said. Or Old Reedy Creek Road could be realigned so the fence doesn't cross it, he said.

"Those are just two options. I’m sure the parks folks have other options we’ll talk about," he said.

Some fence opponents dispute RDU's stance that the property is owned by the airport. But RDU spokeswoman Crystal Feldman said state law gives the airport control over the land, even if it's jointly titled to nearby cities and counties.

"The people opposed to the airport protecting its land from trespassers either do not have a clear grasp of the state law or are intentionally disregarding it," Feldman said in an email to WRAL News.

Still, Lew said, the land isn't being used by RDU, so the public should get its benefit.

"We’ve got this beautiful, extremely unique forest in the middle of a metropolitan area," she said. "It’s about everybody that lives here, visits here, animals and people."

The 8-foot-high, chain-link fence, topped with barbed wire, would cost $6 million to $8 million, Sandifer said. The first phase, which the airport board could approve as early as next month, would cover 8 miles.