UNITED NATIONS - U.S. President Barack Obama makes his United Nations debut on Wednesday, hoping his international star power can throw new muscle behind a policy agenda that is already struggling to deliver.

Obama's maiden speech to the U.N. General Assembly is expected to highlight the new tone he has brought to U.S. foreign policy, stressing cooperation and consultation over the strident unilateralism of his predecessor, George W. Bush.

A U.S. official said Obama would discuss what his administration is doing to confront global challenges and will ask other countries to live up to their responsibility to act as well.

While his global popularity all but assures Obama a warm U.N. welcome, the audience will be studded with reminders of past problems and future perils.

Iranian President Mahmoud Ahmadinejad, Libyan leader Muammar Gaddafi and Zimbabwe's President Robert Mugabe are among the leaders also due to address the gathering -- a chorus of skeptics who will likely seek to undercut Obama's star turn.

Also due to make speeches during the week are Israeli Prime Minister Benjamin Netanyahu and Palestinian President Mahmoud Abbas, who have both already rebuffed Obama's efforts to reinvigorate stalled Middle East peace talks.

Obama's moment in the U.N. spotlight will at least afford him relief from troubles at home, where his approval ratings have slipped amid bitter debate over his chief domestic policy priority -- reforming U.S. healthcare.

Obama, who later this week moves on to host a Group of 20 summit in Pittsburgh, will likely call for further cooperation to buttress the weak global economy and fight climate change.

He will detail the priorities of non-proliferation, peace and security, climate change, and global growth and development, and underscore America's fundamental commitment to universal values -- and challenge others in the United Nations to do the same, said a senior U.S. official, who gave a preview of the speech on condition of anonymity.

Iran's Ahmadinejad has rocked the boat before at the General Assembly, and his speech later on Wednesday will likely be the sharpest counterpoint to Obama's address.

Iran is due to hold direct talks next month with the United States and other international powers concerned about its nuclear ambitions.

But Ahmadinejad recently repeated that Tehran will never drop its nuclear program and said again that the Holocaust was a lie, raising the stakes before next month's talks and spurring Germany to threaten a walkout if he repeats it again in his U.N. speech this week.

The United States and other members of the negotiating group -- the five permanent members of the U.N. Security Council and Germany -- are due to meet on the sidelines of the General Assembly on Wednesday.

SEEKING SUPPORT FOR AFGHAN WAR

The U.S. leader is expected to seek to shore up support for the war in Afghanistan, where U.S. combat deaths have risen as a resurgent Taliban has confounded efforts to stabilize the country.

Obama can be expected to reassert the U.S. commitment to Afghanistan and to aiding neighboring Pakistan in its fight against Islamic militancy. But whether that will be enough to win deeper international backing remains to be seen.
Obama's global entreaties have already come up short on at least one count.

Despite hard lobbying before this week's U.N. meeting, U.S. diplomats were unable to broker a breakthrough between Israel and the Palestinians, prolonging a standoff that has bedeviled generations of U.S. leaders.

Obama held talks and a photo opportunity with Israeli Prime Minister Benjamin Netanyahu and Palestinian President Mahmoud Abbas on Tuesday, but anyone hoping for concrete signs of progress was disappointed.

Libya's Gaddafi follows Obama to the podium on Wednesday for his first U.N. speech, one that could inflame U.S. emotions over the Lockerbie bombing following Scotland's release of a Libyan official accused in the 1988 attack.

Not in the New York audience will be North Korea's reclusive president, Kim Jong-il -- another fixture in former President George W. Bush's axis of evil who still confounds policymakers in the Obama administration.

North Korea could figure in Obama's push for tougher global safeguards against nuclear proliferation. Obama will likely press Pyongyang to return to suspended six-party talks on its own nuclear program. (Additional reporting by Caren Bohan; Editing by Doina Chiacu)

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