After spending the night at Buckingham Palace, US President Barack Obama travelled to 10 Downing Street where he and Cameron held talks before a midday news conference.

Later Obama delivers a speech at Westminster Hall laying out the main theme of his trip – that the United States and Europe must play a lead role together in global security.

Libya presents a thorny challenge. After initially leading an air campaign against Gaddafi's forces, the United States has assumed a lower profile, with Obama trying to avoid another expensive foreign entanglement.

Britain, France and other NATO members have taken the lead but face just as many financial hardships at home as the United States, and there are some suggestions that the allies would like Washington to do more.

That does not appear to be on the horizon, however.

Ben Rhodes, a deputy White House national security adviser, told reporters the United States would not mind if Britain were to send its attack helicopters in support of the Libyan rebels, as London is contemplating.

"There's a lot of ways that the US is contributing diplomatically through support to the opposition and through support to the military effort. And we're satisfied that we'll continue to do so and that that is playing an incredibly important role in the operation," Rhodes said.

Obama is on a week-long, four-nation Europe tour and Wednesday promises to offer the most substance after a light-hearted visit to Ireland and a day in London filled with pomp at Buckingham Palace.

Obama is feeling the tug of home. A tornado killed at least 118 people in Missouri and he plans to visit there on Sunday upon his return.