Flames are seen after a fire erupted following an explosion caused by a gas leak in an apartment building in Rosario city, in the province of Santa Fe, August 6, 2013. [Photo/Agencies]

BUENOS AIRES - At least nine people were killed and another 63 injured when a boiler exploded Tuesday in an apartment building in Argentina's second-largest city of Rosario, local authorities said.

About 21 people remained missing, healthcare, according to police and fire officials, as well as the Civil Defense Office in Rosario, some 360 km north of Buenos Aires.

The official number of fatalities, injuries and missing persons can rise as time goes by, they said minutes after 4 pm local time (1900 GMT).  

The blast occurred at 9:30 am local time (1230 GMT) in the building located at the intersection of Orono Boulevard and Salta Street.

Little information has been released about the victims, except that two women were among the dead, one believed to be 21 years old.

Fire Chief Ernesto Lopez Salvatierra said a boiler in the building located at 2145 Salta Street exploded, causing walls to collapse and sparking a large fire.

Rosario's Civil Defense confirmed the fire was extinguished around 2:45 pm local time (1745 GMT)

Municipal Health Secretary Leonardo Caruana reaffirmed that "so far, nine people have been killed," adding that "the number of injured rose to 63 and they were taken to public as well as private hospitals."

Caruana also said "we are searching with intensity for several people whose relatives, friends or coworkers affirm they are missing."

Argentina's Defense Minister Agustin Rossi and Security Minister Sergio Berni have arrived at the site. They, together with Santa Fe Provincial Gov. Antonio Bonfatti and Rosario Mayor Monica Fein, were coordinating the medical attention for the injured and the rescue efforts for those still trapped in the debris.

Special teams from the Federal Police with trained dogs to find survivors are working to find people buried under the rubble of the 10-story building that completely collapsed.

The people living in a two-block radius of the blast were evacuated and have not been allowed to return yet.

Eleven schools suspended classes minutes after the accident and they will be closed for the next two days.

A neighborhood resident, graphic designer Florencia Papucci, said "there was a tremendous explosion," adding that "you could feel the tremor, like an earthquake.

As night fell in Rosario, emergency floodflights powered by mobile electric generators were called in to light the scene for rescue workers.

Meanwhile, thousands of police officers from Santa Fe Province, the National Gendarmerie and the Navy Prefecture surrounded the area to prevent looting. Earlier in the day, several delinquents pilfered the belongings of firefighters working at the site.  

In a phone interview with Xinhua at about 5:30 pm (2030 GMT), Caruana said that "a total of 26 ambulances are still working in the area. These ambulances were deployed from the first instant at the accident site."

"We continue with the state of maximum alert in the Integrated System of Medical Emergencies, which operates all the public and private health system in Rosario," he said.

Electricity and gas were cut off to a sizable area of the city as a preventive measure.