Libya is tense for the anniversary of the killing of the US ambassador Chris Stevens on Wednesday after a jihadist group posted an online picture of a missile and asked respondents to its Facebook page to "vote" for one of three targets to fire it at.

The previously unknown group, Islamic Emirate of Libya, asks respondents to help choose the target, listing Libya's general national congress, "embassies" and the Thunderbolt special forces brigade in Benghazi with a caption: "Where would you like us to put this missile?"

It added: "With the approach of the global day of horror for the infidels and the Apostates, 11 September, there will be a bombing."

An earlier posting by the group specified that the US embassy was among the three targets.

Early on Wednesday a powerful bomb ripped through a building in Benghazi once used as the US consulate under the reign of Libya's former king, Idris, 50 years ago.

The building, which houses offices of Libya's foreign ministry, was left a blazing ruin. It caused injuries but no deaths, detonating before staff arrived at work.

Diplomats are on alert in Tripoli, with some embassies and foreign companies on lockdown. "I'm not going out, just staying at home, staying low," said one American in the capital.

The United States has moved military assets close to Libya for the day, with 250 marines flown from its base in Spain to a naval air station in Sigonella in Italy, and a further 300 marines on alert on the amphibious assault ship USS San Antonia in the Mediterranean.

Striking army units and tribal groups continue a blockade of oil ports, reducing exports to a 10th of normal levels, and Tripoli is facing petrol shortages, power outages and water cuts.

The leader of the Muslim-Brotherhood-affiliated Justice and Construction party, Mohammed Sawan, has demanded that the prime minister, Ali Zaidan, step down.

"Nothing is happening. It is almost total shutdown. We are seeing problems with water and electricity," Sawan said. "That makes it clear to the normal person that the government is not doing its job."

The US, Britain, France and Italy meanwhile have taken the unprecedented step of issuing a joint statement backing the prime minister. "We support the efforts of Prime Minister Zaidan and the Libyan government to resolve disruptions," the statement said.