TOKYO (AFP) - A Japanese engineering firm said on Sunday that 10 of its Japanese and seven of its foreign workers remained unaccounted for at an Algerian gas plant seized by Islamist militants, adding the situation was "grave".

The Malaysian foreign ministry, quoting the firm, JGC Corp., said that two of its nationals were among the seven unaccounted for, and there was a "worrying possibility" that one of them was dead. The other three Malaysians who had been working at the plant had been confirmed safe.

JGC said it had confirmed the safety of 61 of 78 workers after Algerian troops stormed the remote gas plant Saturday to end the hostage crisis in which Algerian authorities said 23 foreigners and Algerians were killed.

"We have newly confirmed the safety of 41 of our workers but the safety of the remaining 10 Japanese and seven foreign workers is yet to be confirmed," JGC spokesman Takeshi Endo told reporters.

"We are taking very gravely the information, which has been announced by the government, that a number of Japanese have been killed," he said.

Of the 17 Japanese working at the plant, seven have been confirmed as safe by the company.

"We acknowledge that we are in a grave situation, judging from the government information and information we have obtained from our office in Algeria," Mr Endo said.

JGC has not confirmed the nationalities of the foreign workers who are unaccounted for, although the company has said Algerian, Malaysian and Philippine nationals were among its workers at the plant.

"We cannot give you the nationalities of the seven foreigners as we are still in the process of completely identifying them," JGC spokesman Ryouske Hoshijima told AFP.

But in Kuala Lumpur, the foreign ministry said JGC had informed it that "there is a worrying possibility that one of the two Malaysians who are still unaccounted for is dead whilst the fate of the other Malaysian is still unknown".

Japan's Chief Cabinet Secretary Yoshihide Suga said his government had yet to confirm independently any Japanese death in the crisis, although the Algerian government had informed Tokyo about an unspecified number of Japanese deaths.

Prime Minister Shinzo Abe said earlier Sunday he had received "severe information" from Algerian Prime Minister Abdelmalek Sellal when they talked on the telephone about the end of the stand-off.

Tokyo had asked the Algerian government to prioritise saving the lives of hostages.

"It is regrettable that I was given sombre information," Abe was quoted by Deputy Chief Cabinet Secretary Hiroshige Seko as telling Sellal.

"The government and JGC will cooperate in ascertaining the safety of Japanese," Suga, the top government spokesman, said as Japanese vice foreign minister Minoru Kiuchi and JGC president Koichi Kawana stood by in the capital Algiers.

The desert plant in In Amenas in located about 1,000 kilometres southeast of Algiers.

"We want to reach the site as soon as possible by checking safety as there is the danger of landmines being laid there," Suga said.

The Algerian interior ministry said 23 foreigners and Algerians died during the hostage crisis that began when the Al-Qaeda-linked gunmen attacked the In Amenas facility deep in the Sahara desert at dawn on Wednesday.

Twenty-one hostages died during the siege after two people were killed on a bus before the kidnappers took hundreds of workers hostage when they overran the plant.

Thirty-two kidnappers were also killed, and special forces were able to free "685 Algerian workers and 107 foreigners", the ministry said.