The United States should consider targeting Al-Qaeda extremists in Yemen with armed drones, air strikes, or covert operations, Senate Armed Services Committee chairman Carl Levin said Wednesday.

"Most options ought to be on the table," short of invasion by US forces, the Democratic lawmaker from Michigan said on a conference call with reporters after a trip to Afghanistan and Pakistan.

"It's everything from physical actions that could be accomplished in terms of use of drones or air attacks, it could be clandestine actions," said Levin, a top Senate ally of US President Barack Obama.

Yemen's capacity to combat extremism has emerged as a hot button political issue in Washington following the thwarted Christmas Day attack on a US airliner heading for Detroit, which was allegedly planned in the Arab country.

The lawmaker said Yemen's government "understands" the nature of the threat from Islamist extremists and that increased US military aid to the government in Sanaa was "clearly something that should be on the list."

Asked whether the United States would act with or without Yemen's approval, Levin replied "it would be expected that they would either agree privately, or accept without objection privately."

But the senator, speaking by telephone from Dubai airport, underlined that "if we really had high-value targets" then Washington would regard it as its right under international law to "unilaterally go after that target."

After the thwarted attack on a Northwest airliner at Christmas, the United States and Britain announced plans to fund counter-extremism police in Yemen, the base of Al-Qaeda in the Arabian peninsula, which is blamed for the plot.

But with an eye on the unstable political environment in Yemen and neighboring nations, Obama is making clear that he does not envisage openly deploying US troops to the country.

Levin said he did not know what options the Pentagon was considering but expected to find out more at a forthcoming briefing.