The fire department’s citywide response time has improved significantly since it implemented a new dispatch system a month ago, but union officials are opposing the change, saying the rush to fires is actually jeopardizing public safety.

The new program has exposed a sharp division between the fire commissioner and local leaders, who exchanged barbs yesterday following an announcement that average response times have plummeted to their lowest level in years. In the first indication of the program’s effectiveness since it was launched across the city, fire officials told The New York Sun that the department responded to structural fires 18 seconds faster between June 16 and July 15 versus the same period last year.

Emergency dispatchers now send out trucks as soon as they determine the address and nature of the fire, rather than waiting to gather more information first, as they did in the past. Commissioner Nicholas Scoppetta said this relatively simple change has led to the department’s sharpest improvement in response time since he took office in 2002. It currently takes firefighters an average of four minutes and nine seconds to arrive at structural fires after dispatchers receive a call.

“Every second counts, and fires can develop and double in size in a matter of seconds depending on the circumstances,” Mr. Scoppetta said. “These are remarkable reductions in response times. They are lifesaving.”

Union officials and some City Council members claim that firefighters are more prone to make mistakes if they are sent out with incomplete information. Critical details that affect planning, such as which floor a fire is on, are relayed to responders en route under the new system. Between getting their men geared up and navigating a speeding truck through traffic, officers could be too distracted to assess the incoming information properly, the president of the Uniformed Fire Officers Association, John McDonnell, said.

“A few seconds that would be interpreted as a delay by the department, we interpret as productive and helpful,” he said. “There’s a great risk involved here, and the department seems willing to accept it, and we’re not.”

His sentiment was echoed by the president of the Uniformed Firefighters’ Association, Steve Cassidy, who called the new response time a “deliberately misleading” measure of effectiveness.

Mr. Scoppetta dismissed the unions’ position as politically motivated.

“I can’t understand anyone who would be critical of getting to fires and other emergencies faster than they were before,” he said. “They tend to disagree with whatever headquarters says.”

Opponents pointed to several incidents in which rescues were botched under the new system. In February, when the program was being tested in Queens, an 87-year-old woman died because the department dispatched an inadequate number of trucks, according to union officials. A firefighter was hospitalized a day later after his team was initially sent to the wrong street in Jamaica, they said.

Council Member Leroy Comrie, who represents part of Queens, said the rush protocol is inappropriate for a borough like his, where similarly named streets and callers’ imperfect English can be confusing.

“I’m still very fearful of this system, and I hope they’ve gotten the kinks out,” he said. “There’s a challenge for a New York dispatcher to understand and interpret all those dialects, especially from a person in distress trying to report a fire.”

Council Member Peter Vallone, chairman of the Public Safety Committee, has expressed skepticism about the program. So has Council Member Tony Avella, who serves on the Fire and Criminal Justices Services Committee and said he is inclined to trust the unions.

“The fire department brass is reflecting the mayor’s overall management style, which is to run the city like a business,” he said. “In this case, it affects people’s lives.”

A spokesman for Mayor Bloomberg, Jason Post, expressed support for the new protocol, calling it an “innovation.”

While the response time for structural fires decreased by 18 seconds overall, the benefit varied across the city. Queens improved the most, dropping to an average of 4:30 from 4:59. Staten Island, by contrast, saw its time increase by eight seconds, to 4:36. Fire department officials said they could not explain Staten Island’s increase, but noted that in general firehouse concentrations and geography varies across boroughs.