The Foreign Office advises Britons to leave the Libyan city of Benghazi immediately because of a "specific and imminent terrorist threat".
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The unusually forceful advice from the British authorities was matched by similar messages from the German and Dutch governments to their own citizens.
On Wednesday night intelligence sources received what were considered credible threats that "terrorist organisations are intending to attack oil fields in the country in an 'Algerian-style' attack."
The warning was reportedly corroborated by US and Israeli intelligence.
The Libyan government was reported to consider the British advice as an over-reaction, but there were some reports coming out of Libya that al-Qaeda linked militants may be surrounding Benghazi on Thursday evening.
Algerian government intelligence sources said that two or three of the kidnappers captured alive after last week's In Amenas gas field hostage crisis were also involved in the attack on 11 September last year in Benghazi which led to the death of US Ambassador Chris Stevens and three of his colleagues.
Libyans have blamed that attack on the Ansar al-Sharia group which is based in the east of Libya.
However one independent intelligence analysts suggested to Channel 4 News that former Libyan rebel groups, Islamist or otherwise, may just be "trying to shake things up a bit" and make their presence felt to ensure they get their share of Libya's oil money.
Channel 4 News International Editor Lindsey Hilsum says the picture in the region remains extremely complicated: "large numbers of Libyans fought for al-Qaeda against the Americans in Iraq, while the man believed to have been behind the Algeria hostage crisis - former al-Qaeda leader Mokhtar bil-Mokhtar - is said to have been seen recruiting in the Libyan town of Sirte."
Western governments remain concerned about who now has control of the large amounts of weaponry and ammunition that went missing during the fall of the Gaddafi regime in Libya.
In response to the heightened security situation the French have sent extra special forces to guard a uraniam mine in neighbouring Niger which supplies most of the uranium for the French nuclear power industry. Four French people who were kidnapped from the mine in 2010 are still being held by al-Qaeda in the Islamic Maghreb.
It is unclear how many westerners remain in the city of Benghazi. Flights out of the city were overbooked on Thursday, and the Libya Herald newspaper reported that the British School in the city would be closed all next week but planned to reopen on 2 February.
September's fatal attack on the US embassy staff led to US citizens leaving Benghazi in droves, while French doctors working in the city's hospitals left when their government began its attack on militants in northern Mali, two weeks ago.
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- Mali - a beginner's guide Lindsey Hilsum blogs on International Affairs