Mayor Bloomberg is threatening to cancel all public school trips to the United Nations and alert the public to the perils of entering the headquarters building if Secretary-General Ban fails to repair conditions that city officials say make for a fire trap.

Mr. Bloomberg’s letter follows a dispute that erupted when the mayor’s sister, the city commissioner for the United Nations, Marjorie Tiven, addressed similar safety issues with the U.N. management chief, Alicia Barcena. At the time, Ms. Tiven detailed hundreds of specific safety code violations, saying the level of cooperation by U.N. officials in addressing them was “unacceptable.”

Some U.N. officials blame the dispute, first reported by The New York Sun, on sensitivity of the Bloomberg administration after facing safety failures around the city, including the death of two firefighters in August in the partially dismantled Deutsche Bank building near ground zero and the bursting of a steam pipe on 42nd Street in July.

“We stand by our letter and, while there has been some progress, there are still significant differences in the approach and timeline for fixing these problems,” Mr. Bloomberg’s spokesman, Stuart Loeser, told the Sun.

Since Ms. Tiven’s letter to Ms. Barcena last summer, “More than six months have passed,” Mr. Bloomberg wrote to Mr. Ban in an October 30 letter, first reported yesterday by the Washington Times. “The New York City Fire Department estimates that less that twenty percent of the 866 previously issued violations have been cured,” according to the letter, which specifies seven “key steps” the U.N. must take to provide safety for employees and visitors.

“The U.N. estimates that 40% of the directives have been cured,” Ms. Barcena wrote in a November 5 answer to Mr. Bloomberg’s letter, adding, “We agree with you that more can be done.”

“We are working toward the deadlines we have agreed with New York,” the director of the U.N.’s capital master plan, Michael Adlerstein, told the Sun yesterday. He noted that the U.N. recently appointed ten fire inspectors to roam the building in search of fire hazards. This is a “relatively common” procedure to address safety issues in city buildings where the fire safety systems are down, he added. “The city needs to be assured that the building is safe,” he said.

U.N. officials point to an incident that made world headlines last week: just as Israel’s U.N. ambassador, Dan Gillerman, was set to address a General Assembly committee debating the Human Rights Council, a fire alarm went off. The assembled diplomats scrambled for the doors, leaving Mr. Gillerman to wonder half jokingly whether the U.N. was once again attempting to silence his country.

As it turned out, the alarm went off because a steam pipe in one of the building’s basements erupted. According to U.N. officials, their own fire officers quickly identified the problem, and by the time a city fire truck arrived in front of the building, the steam pipe failure was addressed. More than anything, they said, the incident demonstrated how their building is crumbling, and how the Capital Master Plan, an ambitious $1.9 million project to renovate the building, must be quickly launched.

Once completing the renovation plan, which in its current version would be launched early next year and completed by 2013, “the complex will be totally compliant with all New York City fire and safety codes,” Ms. Barcena wrote. As the General Assembly has been slow to approve the plan, however, the project may be delayed further, perhaps adding to its costs. Meanwhile, at least according to Mr. Bloomberg’s letter, employees and visitors are at risk.

Mr. Adlerstein and other officials say that 385 emergency signs and directives were installed recently, as were 2,000 new sprinkler heads. But in his letter, Mayor Bloomberg wrote that among the city’s demands, the U.N. must launch a “separation and compartmentation” project, hire city-certified fire directors, install additional smoke detectors, and shut down an antiquated, dangerous fan system. A plan to fulfill all these tasks should be presented by January 2008, and they should be implemented by March.

These steps “Must be taken to protect the public safety,” Mr. Bloomberg wrote.

If the United Nations fails to meet the deadlines, he added, the city “will be forced to direct the cessation of all public school visits to the United Nations. If warranted, the city will take additional action, including notifying the public of the outstanding hazards” — and of the city’s attempts to address them.