TOKYO (REUTERS) - Nine Japanese nationals were killed during a four-day siege at a gas plant in Algeria, where the number of hostages killed had already reached at least 48, a Japanese government source said on Monday.

"We've received information from the Algerian government that nine Japanese are dead," the official, who declined to be identified pending the official announcement, told Reuters.

The Japanese government and engineering firm JGC Corp , which had several dozen employees working at theplant, have so far said only that 10 Japanese workers remained

unaccounted for. Neither would confirm media reports of casualties.

JGC is due to give an update regarding its staff in Algeria at 0600 GMT (1400 Singapore time).

The deadly siege, one of the worst international hostage crises in decades, was claimed by a veteran Islamist fighter on behalf of al Qaeda.