A steam pipe ruptured underneath a street in the Flatiron district of Manhattan on Thursday morning, producing a thick geyser of white smoke and debris that contained asbestos, the authorities said.

Test results that showed there was asbestos in the steam line raised concerns about the long-term effect of exposure through debris, Mayor Bill de Blasio said at an afternoon news conference.

A total of 49 buildings were evacuated, and Fifth Avenue between 19th and 22nd Streets remained closed, he said. Residents in the area could be displaced for a few days, though he said it’s possible buildings farthest from the blast could be cleared as early as Thursday evening.

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Eric F. Phillips, press secretary for the mayor, said Con Edison would pay for hotel accommodations through its claims process. About 500 people were displaced and nearly 250 residential units affected, the press office said.

While five people received “very minor injuries” when the 20-inch pipe burst around 6:40 a.m., no one was seriously injured, according to the New York Fire Department, which initially evacuated people from 28 buildings near the rupture on 21st Street and Fifth Avenue.

The city is still investigating the cause of the explosion, Mr. de Blasio said. Gov. Andrew M. Cuomo said that the state’s Department of Public Service would also investigate the explosion.

Mr. de Blasio urged people whose clothes were contaminated with debris from the blast to remove them, bag them and take a shower. Clothes can be turned in at a site set up by Con Edison at 22nd Street and Broadway.

There is concern over whether asbestos entered nearby buildings through air-conditioning systems, and officials will work to determine when they will be safe to enter, the mayor said.

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