There were no immediate reports of injuries. Buildings along several blocks of Fifth Avenue have been evacuated as a precaution.
The steam leak was reported at around 6:40 a.m. on Thursday.
Broadcast images showed a large plume of white steam billowing high into the air and the red flashing lights of responder vehicles.
WABC says the blast blew a hole in the middle of Fifth Avenue, sending chunks of asphalt into the street. It says there also were manhole explosions from West 19th Street to West 21st streets.
Some subway trains were bypassing the area.