Television microphones picked up the US commander in Afghanistan, General David Petraeus, joking with Defense Secretary Robert Gates today about possible military intervention in Libya.

"Welcome back sir, how are things?" Petraeus can be heard saying as he greets Gates on the Kabul airport runway, according to a transcript provided to AFP by the US television crew.

"Well, I think you have a bigger plane than normal, you're going to launch some attacks on Libya or something?" Petraeus jokes.

"Yeah, exactly," Gates replies.

The light-hearted exchange came after a number of cautious statements from Gates and other top officers over the crisis in Libya, suggesting the US military has little enthusiasm for intervention in that country -- despite calls from some European allies and American lawmakers.

The audio of their exchange was provided to American television networks by a pool crew covering the Pentagon chief's two-day trip to Afghanistan.

At a press conference later on Monday in Kabul, Gates said the Pentagon was preparing a range of possible options for President Barack Obama but that any military action would have to have international backing.

Gates last week dismissed what he called "loose talk" about possible armed intervention and told members of Congress that setting up a no-fly zone was a major operation that would require bombing raids in Libya to knock out air defences.