BRUSSELS - The Obama administration challenged five key military allies yesterday to take on a greater share of the NATO-led air campaign against Moammar Gadhafi's forces, illustrating the strains of a three-month intervention in Libya that has no time frame for an exit.

Defense Secretary Robert Gates delivered the pointed message in a goodbye to his counterparts from NATO ahead of his retirement next month, senior American and British officials said. But none of the nations that was challenged pledged to do more.

The pressure on Germany, Poland, Spain, Turkey and Netherlands comes as the alliance continues with intensified air strikes on Libya's capital.

"Secretary Gates was very blunt," said Liam Fox, the defense secretary of Britain, which along with France has led the mission to protect Libyan civilians from Gadhafi's troops.