'None of us can be safe if we walk away from that country,' Brown said, amid wavering from other European allies after a disputed election and deadly fighting against the Taliban.

Brown also called on world leaders to join a December summit in Copenhagen that is aimed at hammering out a new global treaty to tackle climate change. He argued only the presence of world leaders could assure a strong outcome.

'Despite the progress we have made, the road to a successful outcome in Copenhagen is not assured,' Brown said.

Brown said wealthy nations had to commit more funds to help developing nations adapt to climate change, as well as more aid to keep the world's poorest from plunging even further into poverty.

Brown said the Group of 20 nations summit beginning Thursday in Pittsburgh, Pennsylvania, should agree to a 'global compact for jobs and growth' and he warned against governments pulling back too soon from public spending measures to stabilize their economies.

'We do not and must not turn off the life support for our economy prematurely,' Brown said.