Alarmed by developments in Libya, the United States has moved 200 troops to a base in Sicily so they can respond more quickly if the US needs to evacuate its embassy in Tripoli.
The troop move is the latest acknowledgment from the Obama administration that three years after a Nato bombing campaign helped topple the government of Muammar Gaddafi, conditions are deteriorating and security concerns previously confined to Benghazi and Libya's east have spread to Tripoli and the west.
Washington worries that Islamist militias could easily close Tripoli's airport, complicating any effort to evacuate US diplomats if the situation deteriorates further.
Militias are also in position to seize control of Libya's other airports, including the one in Benghazi, where militants in 2012 attacked US diplomatic facilities, killing four Americans including the ambassador, Christopher Stevens.
One senior diplomat in Tripoli said US officials were living under severe security restrictions. "You have to have a new normal," said the diplomat. "We're built to hunker down."
The fluidity of the security situation in Tripoli was in evidence on Friday, as Algeria reportedly sent in members of its special forces and a military plane to evacuate its ambassador and staff after militants threatened them.
But the apparent impetus for the US troop movement was the release last week of Jordan's ambassador to Libya, Fawaz al-Itan, after 28 days as a hostage, possibly in exchange for Jordan's release from jail of a top Libyan Islamist. Officials fear the deal might encourage more kidnappings. How long the US troops will remain in Sicily was unknown.
The central government's lack of control over its fragmented military was never more evident than on Friday, when fierce fighting gripped Benghazi after a key commander in the 2011 uprising, General Khalifa Hifter, commandeered government troops and air power to attack Islamist militias.
The Libyan army declared a no fly zone over Benghazi yesterday. The army said its forces would shoot down any plane violating the ban, which applies to Benghazi and its suburbs. The ban applies "until further notice."
The central government said it had not authorised the attack by Hifter, who was a well-known commander during Libya's incursion into neighbouring Chad in the 1980s but left Libya and lived in the US state of Virginia for years before returning as the anti-Gaddafi rebellion gained momentum.