The plant, which is located in the southern Rostov region, released a terse statement saying seven workers were hospitalised following a fire Thursday evening.

But Russian news agencies, citing informed sources, said one worker had died and the other six were critical.

The statement from the Kamensky plant did not provide any information on the severity of the injuries.

The plant however stressed that the fire had been swiftly put out and "no harmful emissions had been released into the atmosphere."

Authorities launched a probe which is headed by a government official who is in charge of production of conventional arms and ammunition at the ministry of industry and trade.

The plant, which is located in the town of Kamensk-Shakhtinsky and describes itself as working to strengthen Russia's defence capabilities, declined to provide further comment.

TASS and Interfax news agencies, citing informed sources, said Friday that one worker had died.

"Doctors could not save him," an informed source told Interfax, adding that the other six were in critical condition.

"Doctors are fighting for their lives."

The chief doctor of the Kamensk-Shakhtinsky hospital, which is treating the workers, declined to comment.

"Neither I, nor my staff will speak to you," he told AFP.

Interfax said that the plant manufactures weapons and ammunition and its sole owner is the ministry of industry and trade.

Citing the informed source, Interfax said that the fire started because workers at the plant violated safety rules while handling "chemical reagents."

Dmitry Kapranov, the ministry official in charge of the probe at the plant, was not available for comment.

His assistant said he had left for Kamensk-Shakhtinsky Thursday evening.