His Al-Qaeda linked group threatened to stage strikes on nations involved in chasing out Islamists from neighbouring Mali, in a stern warning, published by the Mauritanian news agency ANI.
The group said it had tried to negotiate with the Algerian army to seek an "immediate end to the aggression against Muslims in Mali and the liberation of our brothers being held by the crusaders".
It added: "But the Algerian army did not respond, ... preferring to stage an attack which led to the elimination of the hostages".
Defence Minister Jean-Yves Le Drian of France described the hostage-taking as an "act of war" because of the large number of people involved.
"Five terrorists were found still alive this morning" at the plant, where special forces launched a final rescue bid on Saturday that left 18 people dead, including seven hostages, Ennahar television reported.
But "three others are at large", station director Anis Rahmani told AFP.
Security forces inside the In Amenas plant deep in the Sahara desert discovered the bodies of 25 hostages as they combed the sprawling complex, said the private TV channel.
An AFP correspondent at the In Amenas hospital was told 12 of the bodies stored at its morgue were Japanese, after Tokyo said it had no confirmation on the fate of 10 of its nationals who went missing in the 72-hour ordeal.
"In all nine Japanese were killed," one Algerian witness identified as Brahim said on Sunday.
The first three were killed as they tried to escape from a bus taking them to the airport at the outbreak of the militant attack on the plant run by Britain's BP, Norway's Statoil and Sonatrach of Algeria, witnesses said.
"We were all afraid when we heard bursts of gunfire at 5:30 am (0430 GMT) on Wednesday, after we realised that they had just killed our Japanese colleagues who tried to flee from the bus," said Riad, who works for Japan's JGC Corp engineering firm.
The gunmen then took the others to the residential compound, where they had seized hundreds of hostages, he said.
"A terrorist shouted 'open the door!' with a strong north American accent, and opened fire. Two other Japanese died then and we found four other Japanese bodies" in the compound, he added, choking with emotion.
Citing security sources, El Watan newspaper said "30 other bodies", of foreign and Algerian hostages, as well as soldiers, had been found by the special forces at the site.
But a more definitive toll is only expected at 1330 GMT on Monday when Prime Minister Abdelmalek Sellal is due to give a news conference.
Communications Minister Mohamed Said told a radio station: "I fear that it (the toll) may be revised upward", after at least 23 foreigners and Algerians, mostly hostages, were killed since Wednesday.
Thirty-two kidnappers were also killed in the standoff, and the army freed 685 Algerian workers and 107 foreigners, the interior ministry said.
Describing the situation as "tragic", Prime Minister David Cameron said three Britons were killed and another three were believed to be dead, along with a resident of the United Kingdom.
Relatives of Kenneth Whiteside, 59, from Scotland, were "devastated" after hearing an Algerian co-worker claim to have seen him being shot but dying bravely with a smile, Britain's Mail on Sunday reported.
And the mother of survivor Stephen McFaul, 36, from Belfast, told the Sunday Mirror her son will "have nightmares for the rest of his life after the things he saw".
Statoil said the situation remained "unresolved" for five of its employees, and that searches were underway in and around the complex.
A security official told AFP it was believed seven foreigners were executed "in retaliation" on Saturday during the final assault that state TV said also killed 11 militants.
The gunmen, whose leader Belmokhtar is a former Al-Qaeda commander, first killed a Briton and an Algerian on a bus before taking hundreds hostage at the plant.
Most hostages were freed on Thursday in the first rescue operation which was initially widely condemned as hasty, before criticism was focussed on the jihadists.
"The blame for this tragedy rests with the terrorists who carried it out, and the United States condemns their actions in the strongest possible terms," said US President Barack Obama after at least one American had already been confirmed dead.
Among the other hostages killed were at least one Algerian, one Colombian and two Romanians. Those still unaccounted for include five Norwegians, two Americans and two Malaysians.
Monitoring group IntelCenter said the hostage-taking was the largest since the 2008 Mumbai attack, and the biggest by jihadists since hundreds were killed in a Moscow theatre in 2002 and at a school in the Russian town of Beslan in 2004.
French troops advanced Sunday towards Mali's Islamist-held north as Russia and Canada offered to help transport French and African soldiers to boost the Paris-led offensive, and Germany also pledged assistance.
On a visit to the gas plant, Algerian Energy Minister Youcef Yousfi said the complex would be up and running in the next two days, adding the damage was negligible and that security would be strengthened.