Students watch as US President Barack Obama (L) and British Prime Minister David Cameron high-five as they play table tennis against students at the Globe Academy in London, May 24, 2011. [Photo/Agencies]

LONDON - President Barack Obama is plunging back into the complex security debates over Afghanistan, Libya and uprisings in the Middle East, while trying to reassure European allies that they still are valued partners in US foreign policy.

After the two days of celebration and ceremony that opened his European tour, Obama was to hold bilateral meetings Wednesday with British Prime Minister David Cameron and deliver a speech to both houses of Parliament, an address that the White House billed as the centerpiece of the president's four-country, six-day trip.

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Obama's message to allies across Europe, and Britain in particular, will be that their long-standing partnerships remain the cornerstone of America's engagement with the world, even as the president seeks to strengthen US ties with emerging powers such as China and India.

"There is no other alliance that assumes the burdens that we assume on behalf of peace and security and that, again, invests as much as we do in enforcement of international law and in global development," said Ben Rhodes, Obama's deputy national security adviser for strategic communications.

Among the most pressing issues Obama and Cameron will discuss is the bombing campaign against longtime Libyan leader Moammar Gadhafi. NATO has stepped up its military and diplomatic pressure on Gadhafi's regime this week in an effort to jolt the stalemated operation. The coalition launched a withering bombardment on Gadhafi's stronghold in Tripoli on Tuesday, the same day the US said it would allow the Libyan rebels to open an office in Washington.

Obama has said Gadhafi's exit is inevitable. But with the NATO air campaign now in its third month, lawmakers in the US and in Europe are starting to ask when that exit will come.

The White House said Obama would discuss with Cameron ways the international community can boost its support for the Libyan opposition, including funneling them money from frozen Gadhafi assets. There is also keen interest in Britain over US plans to withdraw forces from Afghanistan. Obama is expected to announce the first phase of the withdrawal within weeks, and British military officials have said they will support whatever Obama decides. Britain has 10,000 troops in Afghanistan, second only to the 100,000 US forces there.

Obama and Cameron will jointly honor the sacrifices of their militaries at a barbecue Wednesday hosted by their wives. US and British soldiers will attend, and the two countries will announce a partnership to share resources to help service members and their families.

Obama began his two-day stop in London with a grand royal welcome from Queen Elizabeth II. The president and his wife, Michelle, were greeted in an elaborate arrival ceremony at Buckingham Palace and toasted at a lavish banquet held in their honor.

The Obamas are also staying at the palace while in London as guests of the queen, who is said to have taken a liking to the American couple.