The plan to upgrade the United Nations complex at Turtle Bay involves putting a 35-story building on top of what is listed as a major terrorist target: the Queens Midtown Tunnel.

That raised concerns that the tunnel would become an even more appealing target for terrorists, who might bomb the tunnel to collapse it, bringing down with it the U.N. building – perhaps while it housed the offices of the secretary-general and other high-profile U.N. figures during the renovation of the Secretariat.

The proposed edifice is a 900,000-square-foot “swing space” that would accommodate Secretariat offices during the renovation of the main headquarters building and would be used later as consolidation space for U.N. staff currently spread across the city.

Although the tunnel surfaces at Manhattan around 36th Street and Second Avenue, its underground course enters the island from the East River along 42nd Street and curves south, running along First Avenue until the exit. That curve lies underneath the Robert Moses Playground, a small city park that is the proposed site for the new building. The park is also home to the tunnel’s 10-story ventilation shaft, its source of fresh air.

At the latest meeting of the United Nations Development Corporation, a state-city entity, representatives of the Bloomberg administration confirmed that the building’s footprint would occupy 28,000 square feet of the city park, which means that the southeast portion of the new building’s base would overlap with the tunnel.

A senior terrorism analyst at the Washington-based Investigative Project, Daveed Gartenstein-Ross, said putting up the new building in the planned location would be “unwise.”

Both the United Nations and New York City’s tunnels, Mr. Gartenstein-Ross said, are identified terrorist targets. He pointed to the foiled New York City Landmark Bomb Plot, a project of the blind sheikh Omar Abdel-Rahman, planned as a sequel to the sheikh’s 1993 attack on the World Trade Center. The plot, unraveled between 1993 and 1995, called for simultaneous truck bomb and car-bomb attacks on the United Nations and the Lincoln and Holland tunnels.

Mr. Gartenstein-Ross also said the government has issued advisories about terrorist chatter related to New York City tunnels since the September 11, 2001, terrorist attacks, especially around the time of the Republican Convention.

Because passenger vehicles entering the tunnel are not searched, “it wouldn’t be difficult for a terrorist to drive a car into the tunnel and explode it,” Mr. Gartenstein-Ross said.

In addition to the danger to the U.N. building and the tunnel, another drawback of the plan would be the required increase in security measures around the tunnel, Mr. Gartenstein-Ross said.

“If they get some sort of terrorism warning, and they think there’s a plot to blow up the building, it’s likely there would be massive increases in security in the tunnel. Every driver going through there would be burdened by whatever security precautions they put into place,” the analyst said.

A spokesman for the Metropolitan Transportation Authority, Frank Pasquale, said 80,000 vehicles use the Queens-Midtown Tunnel daily. The MTA does not disclose what security precautions are taken to guard against attacks on the tunnel.

The executive director of the Manhattan Institute’s Center for Tactical Counterterrorism, R. P. Eddy, confirmed that some terrorists view the United Nations as “a high-value target.”

Mr. Eddy, who was senior policy officer to Secretary-General Annan and has worked with the United Nations on security and counterterrorism, pointed to threats against Mr. Annan by Osama bin Laden, including a May 2004 audio recording in which the leader of Al Qaeda offered a bounty of $125,000 in gold for the secretary-general’s murder.

Mr. Eddy said that, regardless of the feasibility of destroying the new U.N. building through an attack on the tunnel, the mere appearance of that possibility was something to avoid.

“Either way, you’re potentially adding even more attraction to the tunnel as a target, and that in and of itself is a problem,” he said. “Even if the engineering’s not right, the perception that it could actually damage the U.N. makes the tunnel more of a target.”

He said the increased value of the tunnel as a target would require increased protection, including restrictions on use of the tunnel. Because of the tunnel’s importance to the circulatory system of city transit, Mr. Eddy said, “Restricting use ends up not just being an inconvenience, but also an economic hardship, and something we have to avoid.”

A spokesman for the city’s Economic Development Corporation, which has been promoting and overseeing the U.N. expansion project, said: “Security is of paramount concern in all of our planning efforts regarding the U.N. project.” The EDC did not provide any security studies related to the new building’s proximity to the tunnel, though the spokesman, Michael Sherman, said architects and engineers were focusing very heavily on the matter.

Some current and former members of the United Nations Development Corporation said they were unaware of any such studies.

One former member, Leo Kayser, said that the corporation had been made aware of the plan to build the 35-story building on land under which the tunnel passes, and that there had been some talk about tunnel security, but that the issue had not received extensive review.

A current member, Jeffrey Wiesenfeld, said the question of the tunnel “hasn’t given me a second thought,” because security is a concern surrounding the United Nations regardless of the tunnel.

State Senator Liz Krueger and Assemblyman Jonathan Bing, the Democratic state legislators whose districts include Turtle Bay, also said they felt the U.N. area and the new building would be terrorist targets independent of the tunnel. Although security is a concern if the new building goes over the tunnel, Ms. Krueger said, more studies would be required to determine potential danger to the community.

The president of the Turtle Bay Association, Bill Curtis, also cited a lack of information in assessing the threat the building might pose to the community. He said association leaders had been told the new building would rest on top of the tunnel but had not been shown detailed plans. When plans are submitted for review, Mr. Curtis said, “I’m sure we’ll be concerned about the factors that lead to security.”

Members of Community Board 6, which has also been working with the city over mitigation for Robert Moses Park, apparently were not informed that the tunnel would run under the new building. The chairman of the board’s Committee on Parks, Landmarks, and Cultural Affairs, Gary Pa push, told The New York Sun: “This is something that has not even been brought up to us … but now that we know about it, we’ll certainly at least ask the city about it.” Putting the new building on top of the tunnel, he said, “is ironic,” because a reason for the project is security. U.N. officials hope to make their offices easier to protect by consolidating operations in various parts of Manhattan at the new building and by upgrading security as the current Secretariat building is renovated.

Also surprised to learn that the building would sit on top of the tunnel was State Senator Serphin Maltese, a Republican of Queens.

“No one had made us aware of it,” Mr. Maltese said, adding: “All this proves is that this requires a hell of a lot of additional study. I will not condone any construction that could very well endanger any of the people of New York City.”

Mr. Maltese also said he was concerned about the effect putting the new U.N. building on top of the tunnel would have on city transportation. “I will be immediately consulting with the transportation committee here in Albany and trying to ascertain what the facts are and whether the U.N. deliberately kept this very critical piece of information from the Legislature,” Mr. Maltese said.

Mr. Maltese, who in Albany has opposed the U.N. expansion, also said “reams of material” had been sent to his office in an attempt to get him to change his position, but “at no time did any of the material they forwarded me say whether the building was over any section of the tunnel.”

Another opponent of the expansion, City Council Member Simcha Felder, a Democrat of Brooklyn, said he, too, was unaware that the new building would rest on top of the tunnel but was not surprised to learn of it.

“It’s not shocking to me whatsoever,” Mr. Felder said.

The United Nations “can’t figure out how to define terrorism, so they can’t figure out what it means to put a building like that on top of a tunnel,” he said. Mr. Felder said the plan was “ludicrous” because “the U.N. should not be on top of a tunnel, jeopardizing the lives of several human beings.”

He also said there was no question that greater security would be required around the tunnel. “It’s going to require a lot of city spending, a lot more money,” he said.

When the project’s proponents say it will not involve any cost to the city, Mr. Felder said, “It’s all baloney.”