Imposing a no-fly zone on Libya could lead to an all-out war, a senior U.S. military figure warned today. America and Britain have been discussing closing the skies above the crisis-torn country to prevent Moammar Gaddaffi carrying out air strikes on opposition forces.

But General James Mattis, commander of U.S. Central Command, said today: ‘It would be a military operation – it wouldn’t be just telling people not to fly airplanes.

'You would have to remove air defence capability in order to establish a no-fly zone, so no illusions here.'

Opposition forces receive training today in the fight against Colonel Gaddafi. It comes as the British government was forced into an embarrassing retreat over its military plans for Libya.

  

Warning: General James Mattis, left, said a no-fly zone over Libya would first require a military operation to destroy the north African nation's air defence systems, while British Prime Minister David Cameron has been forced into an embarrassing retreat over plans for military action in the crisis-torn country.

Desperate: Refugees fleeing the unrest queue up at the Libya-Tunisia border. The UN has warned that the situation 'is reaching crisis point'. Some 15,000 people are expected to cross the border today alone.

Supplies: A convoy of 18 trucks loaded with humanitarian aid bound for Benghazi travels past a Libyan army tank and a mural adulating Gaddafi in the Tripoli suburb of Gaser Ben Ghisher today.

Closing in: The nuclear-powered USS Enterprise, bristling with warplanes, has left pirate-hunting duty off Somalia and is now heading towards Libya

A UN refugee agency spokesman said today that the situation at the Libya-Tunisia border, where more than 75,000 have fled since February 20, is 'reaching crisis point'. A further 15,000 were expected to cross the border today alone. Many of the refugees have little or no money and have been camping out in freezing temperatures in makeshift camps on both sides of the border.

At least one young Egyptian boy was reported to have died from the cold overnight. Other aid officials say humanitarian aid workers are being blocked from reaching western Libya and patients reportedly are being executed in hospitals and struck by hidden gunmen in ambulances. Earlier the U.S. ambassador to the UN, Susan Rice, told CBS: 'The preparations that are under way are for the possibility, we hope not the likelihood, that there could be a real humanitarian disaster in Libya as the situation unfolds.'

She called exile 'an option' for Gaddafi but said he'll also have to answer to the International Criminal Court. She also said it was time for Gaddafi to 'get off that stage', calling him an obstacle to the achievement of peace and stability in Libya. 'One has to question his grip on reality,' she said, echoing her earlier statement that he was 'delusional'.

Delusional: Gaddafi showed no sign of quitting last night as he told reporters: 'They love me, all my people love me. They would die to protect me' Today it emerged that Gaddafi's forces had failed in a bid to retake the rebel-held city closest to the capital, Tripoli.

Witnesses said pro-Gaddafi troops, supported by tanks and anti-aircraft guns, tried to retake Zawiya in six hours of fighting last night, attacking rebel positions from six directions. They said the rebels managed to push back the attackers in Zawiya which is 30 miles west of Tripoli. 'We will not give up Zawiya at any price,' said one witness.