Jihadist leader Mokhtar Belmokhtar, who Chad said was killed in Mali last month, and four members of his group were condemned to death in absentia by an Algerian court on Tuesday.
Four other defendants present in court were jailed for 13 years and fined 1 million dinars (10,000 euros) each, with another two sentenced to one and nine years respectively.
Mohamed Lamine Bencheneb, leader of the Islamist hostage-takers who was killed by Algerian troops during a four-day siege at the In Amenas gas plant in January, was given the death sentence posthumously.
The accused were charged with "creating and belonging to a terrorist group intending to harm the security of the state, and for possession of explosives and the transfer of ammunition and weapons of war."
The six who appeared in court were detained in July 2012, while six others were tried in absentia, one of whom fled after being granted bail in August 2012.
According to the chargesheet, the accused, all from Algeria's southern Sahara region, belonged to a rear base providing logistical support to a jihadist group led by Belmokhtar, an Algerian Al-Qaeda veteran who masterminded the In Amenas assault.
Thirty-seven foreign hostages died during the siege and rescue operation by the Algerian army, while 29 of the Islamist assailants were killed and three captured.
The chargesheet said the accused were in contact with Bencheneb, whose Sahara Movement for Islamic Justice was supplied by Al-Qaeda in the Islamic Maghreb (AQIM) with "arms and explosives for carrying out attacks against oil companies and pipelines."
It said Bencheneb's group was calling for the creation of a Saharan state, to be supported directly by AQIM, Al-Qaeda's regional franchise.
The accused were either from Ouargla, a town 800 kilometres southeast of Algiers, where Bencheneb was based before he joined the Islamist combatants in northern Mali, or Ghardaia, Belmokhtar's birthplace 600 kilometres south of the capital.
Chad has claimed that Belmokhtar died during fighting in the Ifoghas mountains, on Mali's northeastern border with Algeria, in March, but France, which has led the offensive against Islamists in Mali, has yet to confirm his death.