(Adds more background, production data)
ALGIERS, June 28 (Reuters) - Foreign oil workers arereturning to work at Algeria's In Amenas gas plant nearly a yearand a half after the Islamist militant attack there that killed40 people, a source at Algerian state energy company Sonatrachsaid on Saturday.
"Twelve expatriate workers are already at the gas facilityand more are expected progressively," the source said withoutgiving details on their nationalities.
Norway's Statoil and BP, which operate the facility withSonatrach, had demanded improved security before returningworkers. Forty oil contractors, all but one foreign, died inJanuary 2013 after militants took expatriates hostage during afour-day siege that ended when Algerian forces stormed the site.
In Amenas produced about 11.5 percent of Algeria's naturalgas output before the attack and the North African state hasbeen steadily bringing the plant back to full resumption, whichfrees up more of the fuel for export to Europe.
Algeria will hold a new energy bidding round later this yearwhen it hopes to attract more foreign oil companies to helpbolster its stagnant oil and gas production. After the In Amenasattack, security was a major doubt for investors.
Statoil said late last year some of its staff had returnedon a permanent basis to its operational centre at HassiMessaoud, some 700 km (435 miles) to the southeast of Algiersbut that it would take more time to return to In Amenas. (Reporting by Algiers newsroom; writing by Patrick Markey;Editing by Toby Chopra)