"The reason that I thought this trip was important is that I am convinced that many issues that we face at home are not going to be solved as effectively unless we have strong partners abroad," he said. "And unless we get a handle on Iraq and Afghanistan, not only are we going to be less safe, but it's also going to be a huge drain on resources."
Obama said he and Brown discussed a wide range of issues, such as climate change, terrorism and financial markets.
"The prime minister's emphasis - like mine - is on how we can strengthen the trans-Atlantic relationship to solve problems that can't be solved by any single country individually," he said.
The Democratic hopeful seemed relaxed as he strolled down to the prime minister's office at 10 Downing St., pausing briefly to shake the hands of two somewhat startled police officers standing near the door.
He turned to television cameras, smiled, waved and said "Hello," before walking into Number 10. Brown greeted him just inside the door.
Pooled television images showed Brown offering Obama a chair on the Downing Street terrace before the pair settled down for two hours of talks.
They later took a stroll in the sunshine around Horse Guards Parade, the vast open space where military reviews, such as Trooping the Color, are often held. Tourists snapped pictures from a distance while security guards walked ahead of the two men.
The walk offered a photo opportunity with a London backdrop for Obama, whose visit to Britain has been decidedly low-key, particularly after the huge crowds he drew earlier in the week in Germany.
