Plans by Virginia to build a $325 million annex of the National Air & Space Museum at Dulles International Airport have encountered an unexpected obstacle that may threaten the project, officials said yesterday.

Rep. Sidney R. Yates (D-Ill.), chairman of the House Appropriations interior subcommittee, said Virginia officials are not contributing enough to the cost of building the 27-acre annex. Yates suggested reopening the competition for the annex, where the Smithsonian wants to house aircraft that are too large for the museum on the Mall.

Virginia officials believed they had won the right to construct the annex in January, when the Smithsonian Institution's board of regents picked Dulles over Baltimore-Washington International Airport. The competition between Maryland and Virginia was intense because of the expected economic benefits from tourism.

On Monday, Yates inserted a provision in the Interior Department appropriations bill that stripped $1 million in startup funds for the Dulles project. The House approved the bill, which will go to a conference committee of lawmakers from both houses.

Congressional approval, in the form of authorization and appropriation, is required before the project can proceed. The Senate has already authorized the Dulles project, but the House has not.

Yates, noting the Smithsonian's budget crisis and the institution's expansion plans that include a new national American Indian museum on the Mall, said a proposal by Rep. David E. Skaggs (D-Colo.) to build the annex at Stapleton International Airport in Denver would cost $180 million.

"When we are faced with having to provide an appropriation for $325 million and the opportunity exists to erect it for half of that, it's something Congress should look at," Yates said.

According to Virginia's offer, which was approved by the Smithsonian's 17-member board of regents, the museum was to be given 185 acres at Dulles; a $3 million interest-free loan; $40 million in roads, parking and taxiways; and authority to issue $100 million in bonds. Maryland officials made similar offers.

Of the $162 million cost of the first phase of construction, Yates said, Virginia's guarantees would leave the Smithsonian to come up with $19 million, and the federal government might have to finance the bond issue.

The Denver proposal, he said, assumes more of the cost, including money from private sources.

"It then becomes a question of whether an extension of the Smithsonian should be undertaken in an area not close to Washington," Yates said. "If the Smithsonian is expanding, perhaps some of it ought to be in other parts of the country."

Yates wants the House to refrain from taking further action until it considers other plans, including Denver's.

Virginia Education Secretary James W. Dyke Jr., who was instrumental in preparing Virginia's proposal, said yesterday, "The Smithsonian regents have made it clear Dulles is the best site, and we stand behind the commitment we made."

He acknowledged that the question of whether the state or the federal government would pay back the bonds is still unresolved, but said, "We did exactly as the Smithsonian asked us -- to seek the bonding authority."

Madeleine Jacobs, the Smithsonian's chief spokesman, said, "We feel quite comfortable with the depth of the Commonwealth of Virginia's commitment."

She said that the regents voted last month to explore strategies for attracting private money to the Dulles project, so the bonding authority might not be needed.

Furthermore, she said, moving a part of the air museum to another part of the country, such as Denver, would boost the project's cost to more than $500 million because, in effect, a new museum would be created.

"You have to remember, this is an annex, not a separate, free-standing museum that duplicates the function of the National Air & Space Museum," Jacobs said of the facility proposed at Dulles.

Yates's amendment to the Interior appropriations bill caught Virginia and Smithsonian officials by surprise.

Rep. Frank R. Wolf (R-Va.), in whose district the annex would be located, criticized Yates for slipping in the amendment without warning, saying the action violated the subcommittee's "spirit of cooperation and trust."

The annex would house such aircraft as the Enola Gay, the B-29 that dropped an atomic bomb on Hiroshima; the space shuttle Enterprise; and a supersonic Concorde.