New York City has canceled student field trips to the United Nations after the world body failed to make building improvements on its headquarters.

Marjorie Tiven, the city's liaison to the U.N., sent a letter to officials on Monday saying the city had "no choice" but to suspend the school trips.

• Click here to see the letter obtained by FOX News.

Tiven said that although the world body had corrected many of the 866 code violations the Fire Department of New York found during 2006 inspections, it had "delayed or put off indefinitely" the "most serious building corrections."

Among the city's complaints: the world body's failure to install basic fire protection systems on several sublevels of the 1950s-era complex, including the Conference Building, which was cited for fire issues. The U.N. had told inspectors it avoids the building for public tours.

"The City views any violation of the fire and building safety codes as a public safety issue, regardless of whether the violation applies to an area open to visitors rather than an area accessible only to employees," Tiven wrote. "It is not within the United Nations' discretion (or the City's) to assign different values to the lives of tourists in the Headquarters' public areas, versus the lives of United Nations employees and business visitors in the high-rise Secretariat building."

In October 2007, New York City Mayor Michael Bloomberg warned the U.N. that the city would cancel school field trips if the building were not brought up to code. The 1952 building's safety devices have been at the center of a dispute between the city and the U.N. The world body has a $1.8 billion renovation plan in the works that they say will address some of the issues.

New York City Mayor: U.N. Building a Hazard to Schoolchildren

In Tiven's letter — addressed to Angela Kane, the under-secretary-general in the U.N. management department — she cited the failure to install fire-wall separations in the second and third sublevels of the building. Were a fire to occur there, the flames would run nearly the entire length of the headquarters.

Tiven said that although U.N. officials had claimed to install fire-safety walls in the complex's Conference Building to protect it from the General Assembly and the Secretariat buildings, city inspectors found this to be untrue.

Tiven also said the U.N. has ceased direct communication with the city on building and fire safety matters.

The Associated Press contributed to this report.