Work is continuing to identify British victims of the Algerian siege and repatriate their bodies.
So far three of the six UK nationals thought to have died at the In Amenas plant have been formally named.
David Cameron on Tuesday chaired a meeting of the National Security Council as the Government considered increasing logistical support for the French-led military campaign against rebels in neighbouring Mali.
The Prime Minister has insisted the UK is "not seeking a combat role", but armed forces units have reportedly been placed on "high readiness" to deploy.
The developments came after Mr Cameron heralded what he said was a global "generational struggle" against al Qaida-inspired Islamist terrorism in North Africa. He pledged to provide intelligence and counter-terrorism assets to help track down and dismantle the terror network responsible for last week's attack on an Algerian natural gas plant.
A total of 37 foreign workers are believed to have died at the remote desert facility - part-operated by BP - which was overrun by heavily-armed terrorists. Some 29 of the hostage-takers died, while three were captured by Algerian troops during a special forces mission to end the four-day stand off.
Three of the Britons killed have been named as 46-year-old security expert Paul Morgan, systems supervisor Garry Barlow, 49, from Liverpool, and 59-year-old planning manager Kenneth Whiteside, from Glenrothes, Fife. Colombian BP executive Carlos Estrada, who lived in London, is also believed to have died.
BP group chief executive Bob Dudley said the company "feared the worst" for four of its 18 staff members who had not been found safe.
"We have been gravely concerned for these colleagues and feared one or more fatalities among their number," he said.
"It is with great sadness that I now have to say that we fear the worst for them all. We are doing all we can to support their families and ask everyone to show them consideration and to respect their privacy."