Algerian Prime Minister Abdelmalek Sellal on Monday blamed Islamist militants operating out of northern Mali for the four-day hostage crisis at In Amenas gas complex, where at least 37 foreigners and one Algerian worker were killed.

The death toll, revised up from an initial weekend count of 23 hostages slain, followed the discovery of more bodies at the site during a demining operation.

Broadcaster Ennahar reported that 25 more bodies were found Sunday, after Saturday’s final assault to free remaining captives.

Some of the bodies were strapped with explosives.

Mr. Sellal said that seven of the dead hostages had yet to be identified with five foreigners still missing.

Of 32 militants behind the attack, 29 were killed by the Algerian army and three were captured alive, he said.

Mr. Sellal said the gunmen came from the north of neighbouring Mali, where French and Malian government forces are fighting three Islamist factions, including al-Qaeda in Islamic Maghreb (AQIM). The Algerian government had previously said the attackers came from Niger.

Eleven were Tunisians, with the rest coming from Algeria, Egypt, Mali, Mauritania and Canada, he said.

The group that claimed responsibility for the attack had said it wanted to avenge France’s 10-day-old intervention in Mali. Sellal said the operation had been planned by the militants for two months, with the help of a former driver at the gas plant.

Around 130 foreign nationals and several hundred Algerians were working at In Amenas, Algeria’s largest gas field, operated by Britain’s BP, Norway’s Statoil and Algerian state company Sonatrach.

The United States confirmed that three of its citizens were killed in the attack. State Department spokeswoman Victoria Nuland said seven other Americans survived.

“The blame for this tragedy rests with the terrorists who carried it out,” Ms. Nuland said, “and the United States condemns their actions in the strongest possible terms.” Japan confirmed Monday that seven employees of Japanese engineering firm JGC Corp were slain. Prime Minister Shinzo Abe said the terrorist act was “despicable” and “unforgivable.” The Philippines confirmed six dead. Romania has confirmed two victims, and France one.

A Colombian worker with British residency was missing and feared dead. London confirmed three Britons dead and three missing and feared dead.

British Prime Minister David Cameron said Monday that the hostage crisis must lead to closer cooperation with governments of the region to prevent the establishment of a “new terrorist haven on Europe’s doorstep.” Britain would offer intelligence and counterterrorism assets to an “international effort to find and dismantle the network that planned and ordered” the assault on the gas plant.

Mr. Cameron said Britain was prepared to work with governments “across the region” to defeat the terrorist threats that had emerged in countries including Yemen, Somalia, Nigeria, Libya and Mali.

The situation called for a “patient and resolute” approach with “iron resolve,” Mr. Cameron said.

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