"I had a phone call with President Obama this afternoon to talk about the planning we have to do in case this continues and in case (Kadhafi) does terrible things to his own people," Cameron told the BBC's "The One Show."
"I don't think we can stand aside and let that happen," he said. "We have got to prepare for what we might have to do if he goes on brutalising his own people."
The two leaders spoke amid renewed international calls for a no-fly zone as Libya's air force stepped up strikes on front line rebels on Tuesday.
Cameron also responded to complaints that the West was being too slow to respond to Kadhafi's crackdown on rebels by saying that the UN Security Council had acted with unusual speed to impose sanctions.
"We need to keep on doing those things to isolate this man and his regime and say that he has got to go," Cameron said.
The official statements from London and Washington about the call were less revealing than Cameron's remarks.
"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's office issued an almost identical statement.
Washington has been markedly less enthusiastic about the possibility of a no-fly zone than some 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 on Tuesday told reporters on Air Force One that though Washington was actively considering a no-fly zone, such a step was complicated.
"There are complexities and realities involved in the adoption and implementation of a measure like a no-fly zone that we all need to be aware of as we consider it," Carney said.
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", according to the White House statement.
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 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, believed to be MI6 intelligence officers, flew into Libya by helicopter and made their way to an eastern opposition-held city.
But they were rounded up by lightly armed rebels soon after they arrived, reports said.
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 opponent Senator John McCain renewed his call for a no-fly zone on Tuesday.
"As we speak, innocent people of Libya are being massacred from the air," McCain said at a joint press conference with Australian Prime Minister Julia Gillard.
But 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.
Senator Carl Levin, chairman of the Senate Armed Services committee, said that any no-fly zone over Libya would need international support, including from some Arab nations.
The Democrat warned that in absence of that support, American action would risk turning a revolt against Kadhafi into opposition to the United States itself.
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