TOKYO (AFP) - The Japanese firm that helped build a remote Saharan gas plant at the centre of Algeria's bloody hostage crisis once told investors the country was a haven of stability as unrest swept the region.

Engineering company JGC, known in Japan as Nikki, has been thrust into the spotlight by the hostage crisis, which has seen as many as a dozen Japanese nationals killed.

On Sunday, JGC said it could not account for 10 of its Japanese and seven of its foreign workers, as the firm's chief Koichi Kawana left for Algeria with the country's vice minister of foreign affairs Minoru Kiuchi.

The company has 78 employees - including 17 Japanese - at the plant, which was stormed on Wednesday by heavily armed Islamists who said they were acting in retaliation for a French intervention in neighbouring Mali.

The vast site is operated by British Petroleum (BP), Norway's Statoil and Sonatrach of Algeria.

JGC once pointed to Algeria as a relative oasis of calm in a turbulent region as pro-democracy movements - known as the Arab Spring - swept across North Africa and the Middle East.

"Since the end of 2010, the world has witnessed democratic uprisings in Tunisia, Egypt, Libya and other countries in the Middle East and North Africa," the company said in its 2011 annual report, released in July of that year.

Algeria has had a democratically-elected government since the later 1990s.

"JGC has ongoing projects in Algeria, Qatar, Saudi Arabia and Abu Dhabi. As these countries have not experienced noticeable disorder, our projects have proceeded steadily without being affected."

During a bloody final assault by Algerian special forces on Saturday, 32 kidnappers were killed, while the army freed 685 Algerian workers and 107 foreigners, according to Algeria's interior ministry.

Reports have said dozens of hostages were also killed.

Two Algerian survivors of the hostage crisis employed by JGC said nine Japanese workers were killed during the day-long siege.

On Sunday, a hospital source said twelve bodies being held at a morgue in Algeria's In Amenas hospital, where victims were taken, are Japanese. However, the information could not be immediately confirmed.

JGC, which builds chemical and energy plants, particularly in the oil and gas sector, has about 2,200 employees and expect sales of 600 billion yen (S$8.22 billion) in its current fiscal year, ended September.

The firm, founded in 1928, has been expanding its overseas business in recent years, establishing local offices in Indonesia, Vietnam, the Philippines, Singapore, Saudi Arabia and Algeria.

Shares in the company were off 2.05 per cent at 2,580 yen ($29) in afternoon Tokyo trade on Monday.