The National Capital Planning Commission, the planning agency for the federal government in the Washington area, said yesterday it could not endorse the initial plan to build highway bypasses around the city.

Citing potentially harmful environmental effects on federally owned historic and recreational areas, the commission said it could not back any of the six routes proposed by the Maryland and Virginia highway departments for the eastern and western bypasses outside the Capital Beltway.

"Extensive additional study is required to provide an adequate basis for a decision," the commission said.

The commission, which is required to submit comments on the proposal, challenged the premise of bypass proponents that offering another north-south route around Washington would necessarily remove through traffic from the Beltway.

The relatively high number of interchanges being proposed for some of the routes would undoubtedly serve local traffic and not through traffic, the commission said. Commission members want fewer interchanges to discourage local traffic and keep the bypass less crowded to attract through traffic.

Maryland highway officials want the eastern bypass, starting as far south as Interstate 95 in Caroline County, Va., and continuing northeast across the Potomac into Charles and Prince George's counties and ending at Routes 50 and 301. It would cost up to $1.7 billion and be up to 93 miles long.

Virginia is proposing a western bypass, costing about the same and stretching up to 82 miles. It would start at I-95 in either Stafford or Prince William counties and continue past Dulles International Airport, across the river and into Montgomery County or the Frederick area, ending at Interstate 70.

Maryland Gov. William Donald Schaefer and Virginia Gov. L. Douglas Wilder are expected to make a decision on the bypasses within a year. A preliminary study is being fine-tuned, and comments are being solicited through Aug. 8.

The commission is involved because all six routes being considered affect federal property or interests in the region. Four routes cross federal land.

For example, the commission is worried that a new bridge across the Potomac would be visible from Mount Vernon.

Maryland officials say the bridge wouldn't ruin Mount Vernon's historic setting. The panel wants further study on the visual effects of tall structures near Mount Vernon, and a ban on interchanges within 1,000 feet of the river.

The commission also expressed concern about how some bypass routes would affect Prince William National Forest Park, one of the region's largest natural parks. A western bypass near the park and the Route 234 corridor could have grave environmental and traffic consequences, the commission said.

"If that bridge is going to be visible from Mount Vernon . . . and if that bypass is going to damage Prince William Forest Park, we're going to come unglued," said John G. Parsons, the National Park Service's associate regional director and a commission member.

Parsons said a recent five-year Virginia transportation plan included a western bypass near Route 234, suggesting a bypass route has been selected. But Virginia officials said the bypass route was included only as a planning tool.

Maryland and Virginia highway planners attending the commission meeting said that they would study every issue raised by commission members before preparing their final report.

Other issues included proximity to the Manassas National Battlefield Park, the C&O Canal Historical Park and Balls Bluff battlefield and cemetery in Loudoun County.