LONDON (AP) British Foreign Secretary David Miliband on Saturday accused critics of U.S. President Barack Obama of foolishly expecting him to fix the world's woes single-handedly.

In a speech to London's Fabian Society think tank, Miliband said those sniping at Obama failed to understand the scale of challenges facing the international community, or the shifting power centers of global politics.

Obama has been criticized in recent months over his divisive health care plans and a prolonged decision-making process over whether to send more troops to Afghanistan. He also has been chided for making little headway in promoting Middle East peace, and has been dogged by rising unemployment and a sour economy.

''A year on from President Obama's election, people are already questioning why he has not already solved the world's problems. But the whole point of Obama's campaign was that the power and responsibility to change the world is distributed,'' Miliband said.

''It is only through working together citizens, business and government; emerging and existing powers that we can overcome problems too big for any single leader or any single nation. We all have to play a role. That is the real change we need,'' he said.

The 44-year-old Miliband, often touted as a successor to Prime Minister Gordon Brown as leader of Britain's governing Labour Party, said Obama had given the United States a ''new start domestically and internationally.''

''The cynics are in full cry. The waters may not have been parted. But a new start has been made and new agendas set. I remain an optimist about this defiantly transformational administration,'' Miliband said.

Miliband said Afghanistan remained a key challenge. He urged Afghan President Hamid Karzai to root out corruption and strengthen his country's security forces and justice system.

Miliband said Karzai must show ''those fighting for money, status or power ... the best route to such rewards is to side with the Afghan government.''

He said international supporters of Karzai's government must stay the course. He said they would triumph ''when people start switching sides when ordinary Afghans and Afghanistan's neighbors stop hedging their bets out of a fear that the international community will leave prematurely, allowing the Taliban to return.''

Britain has about 9,000 troops in Afghanistan, the second-largest force after the United States.

Miliband warned that Western values of liberal democracy are being seriously challenged by the economic crisis, human rights abuses during the war on terror, and the success of authoritarian China.

''The West's economic, political and moral authority is more contested now than at any time in the last two decades,'' Miliband said. ''Our task is to respect different values, ways of life and points of view, while holding firm to our own view of the good life.''