The Obama administration gave a green light to the first section of a proposed 146-mile transmission line Tuesday, calling it a critical upgrade to the faltering Northeast power grid and saying it will eventually create 2,000 jobs.

Work on a 4.5-mile section of the Susquehanna-Roseland transmission line is expected to begin soon. Starting in Berwick, Pa., and ending in Roseland, N.J., the high-voltage line, capable of carrying up to 500 kilovolts, will replace an existing one that carries 230 kilovolts. “About 95 percent of the route” is to run alongside the existing transmission line, according to a Susquehanna-Roseland project statement.

“This project brings the nation’s electric system one step closer to a modern, 21st-century grid that is safer and more secure,” Interior Secretary Ken Salazar said in a statement.

Conservation groups condemned the administration’s decision to build the wider, $1.2 billion line, which slices through the Appalachian National Scenic Trail, the Middle Delaware National Scenic and Recreational River and the Delaware Water Gap National Recreation Area in Pennsylvania and New Jersey.

PPL Electric Utilities of Allentown, Pa., will build the 101-mile Pennsylvania portion of the line, and Public Service Electric and Gas of Newark will build a 45-mile section. Completion is expected in the summer of 2015.

Jeff Tittel, director of the New Jersey Sierra Club, said allowing the utilities to build “is an outrageous decision” by the National Park Service, the division of the Department of the Interior that was responsible for reviewing and approving the plan and issuing special-use and construction permits.

“They sold out the Delaware Water Gap,” Tittel said. “The Park Service compromised their integrity and violated their own mission, which is to protect public lands, not give them away to utilities.”

He called it a dangerous precedent that could lead to similar actions on public lands such as Yellowstone and Yosemite national parks.

Cinda Waldbuesser, senior program manager for the National Parks Conservation Association, agreed, saying the decision “clearly violates the founding law of national parks, which mandates the agency to ‘conserve the scenery’ and protect park resources from impairment.”

For the project, trees will be cleared and vegetation removed from an area three times as wide as the existing line, the association said. The new towers will be 190 feet tall; the current towers rise 90 feet.

The utilities encountered skeptical Park Service officials when they submitted an application for construction in March 2009. The service looked at several alternative plans and initially said that none would prevent destruction of natural resources on public lands.

Those concerns were eventually offset by a $56 million mitigation package from the utilities to buy and improve other land. Mitigation is typically required by federal agencies for impact that cannot be avoided.

But Tittel said, “You cannot mitigate for the destruction of a national park.”

Salazar saw the matter differently, saying a stronger transmission line will lessen power overloads on existing lines that sometimes leading to brownouts and blackouts in 300 school districts, 100 hospitals and 1.7 million homes.