UNITED NATIONS — The city of New York is suspending all public school visits to the United Nations due to safety concerns in the landmark First Avenue building, according to a letter to top U.N. officials from the city’s liaison to the world body, Marjorie Tiven.

Expressing “profound disappointment” over the United Nations’ failure to make good on pledges to secure the building, Ms. Tiven, who is Mayor Bloomberg’s sister, writes that the city has “no choice” but to suspend all school trips. The suspension comes as the United Nations is preparing for the U.N. General Assembly session later this month.

RELATED: Marjorie Tiven’s Letter (pdf).

The September 8 letter to the U.N. under-secretary-general for management, Angela Kane, is the latest in an escalating tug-of-war between city and U.N. officials over the safety of the U.N. compound, which has violated city building safety codes throughout its 60-year existence, including at least 866 fire and building safety code violations documented by the fire department in recent months.

Although the United Nations is outside the legal jurisdiction of its host country, Ms. Tiven’s office has demanded that the world body comply with the codes, arguing that city first responders would be called upon to save lives in U.N. building during an emergency. The request has led to legal wrangling between the two sides, but recently the United Nations began to work with the city, erecting fire-separation features in parts of the building. U.N. officials say they have spent at least $3 million to comply with the city’s safety demands, funds they say will be wasted when the organization launches its $1.9 billion Capital Master Plan to overhaul the building by 2013.

U.N. officials have recently limited public tours of the building, one of the city’s most popular tourist destinations, confining day trips to areas that are fire-compartmentalized. But Ms. Tiven writes that the move does not address safety concerns for employees working at the 39-floor building and for nontourists who visit the institution, including dozens of heads of state attending this month’s annual General Assembly session.

“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,” Ms. Tiven writes.

“We are confident that the U.N. facilities are very safe,” the executive director of the Capital Master Plan, Michael Adlerstein, who has negotiated with the city over safety issues, told The New York Sun.

“We look forward” to the arrival of the General Assembly delegates later this month, he said, adding that the building is as safe as it has been over the last 60 years.