British government has confirmed Sunday that five British nationals and one resident in Britain have been killed or are unaccounted for as the Algerian hostage crisis drew to a end.

"One British citizen has already been killed in this brutal attack and we now fear the worst for the lives of five others who are not yet accounted for," said British Prime Minister David Cameron.

According to Foreign Secretary William Hague, the six were "in addition to the one fatality that we had already confirmed."

"Our priority now is to work out exactly what has happened to every British national who might have been caught up in this terrible situation," Hague said.

"We're in touch with all of the families concerned and we're working closely with BP and with police forces across the country to give those families the support they need at this very difficult time," he said.

The Algerian government said earlier on Saturday that at least 23 hostages were killed during the stand-off with al Qaeda-linked Islamist gunmen who stormed the gas plant in the Saharan desert on Wednesday.

Local media reports said all 32 militants were killed, while 107 foreign workers and 685 local employees had been freed.