WASHINGTON – US President Barack Obama and British Prime Minister David Cameron agreed Tuesday to plan a "full spectrum" of action on Libya, including a possible no-fly zone, surveillance and a relief effort.

Obama and Cameron spoke amid calls for immediate international action to halt Moamer Kadhafi's crackdown on rebels but with no clear unified world response to the strongman leader's defiance emerging.

"The president and the prime minister agreed to press forward with planning, including at NATO, on the full spectrum of possible responses," the White House said in a statement.

Possible measures included surveillance, humanitarian assistance, enforcement of the arms embargo, and a no-fly zone, the White House said.

Cameron and Obama spoke amid renewed international calls for a no-fly zone as Libya's air force stepped up strikes on front line rebels on Tuesday.

Washington has been markedly less enthusiastic about the possibility of such a step than some of its allies, with some officials noting that it would likely require bombardments of Libya's air defenses.

Britain and France have been drawing up a draft UN Security Council resolution on a no-fly zone which could be presented as early as this week.

But White House spokesman Jay Carney told reporters on Air Force One Tuesday that though Washington was actively considering a no-fly zone, such a step involved "complexities."

Cameron and Obama also agreed that there must be an "immediate end to brutality and violence" in Libya and on the need for Kadhafi to leave power as "quickly as possible."

Obama also thanked Britain for its "partnership" at the United Nations and for its humanitarian assistance to the Libyan people.

The US president's telephone call to Cameron came with the British government under domestic fire over a botched US special forces mission to contact Libyan rebels.

The team, reportedly made up of six soldiers from the elite Special Air Service (SAS) and two diplomats, flew into Libya by helicopter and made their way to the eastern opposition-held city.

But they were rounded up by lightly armed rebels soon after they arrived, reports said.

The diplomats are believed to have been officers from Britain's MI6 foreign intelligence service.

Obama is also facing political pressure at home over Libya with several prominent members of Congress, including some allies, increasingly vocal in their calls for a no-fly zone.

His 2008 Republican presidential foe John McCain has called for the establishment of a no-fly zone. Key Democratic Senator John Kerry, who chairs the Senate Foreign Relations Committee, has also called for such a step if Kadhafi's air force begins killing civilians on a large scale.

But some other important members of Congress have been more circumspect, backing Defense Secretary Robert Gates's caution over the idea of enforcing aerial restrictions over Libya.