U.S. President Barack Obama begins his biggest week yet on the world stage, facing pressure for results on an agenda of rekindling Middle East peace talks, tackling climate change and reshaping financial regulations.

Obama's global starpower remains strong but doubts are emerging about what he can deliver in a week in which he will make his United Nations debut and host a financial summit.

The nuclear dispute with Iran and the Afghanistan war will be among the top issues as Obama begins three days of U.N. meetings on Tuesday. He will lay out his foreign policy vision in his first speech to the U.N. General Assembly on Wednesday.

In the shadow of a financial meltdown that triggered fears of another Great Depression, he will lead a summit of the Group of 20 biggest economies on Thursday and Friday in Pittsburgh.

Obama got a rapturous welcome overseas when he succeeded President George W. Bush eight months ago, winning plaudits for his rhetorical gifts, his multicultural background and his promise to break with Bush's unpopular go-it-alone style.

He has made major strides toward keeping his pledge to restore Washington's reputation abroad, but analysts say he needs more concrete results to show for it.

Russia and China have not agreed to new sanctions on Iran, Europe will not send significantly more troops to Afghanistan and his efforts to jump-start Middle East peace appear set to yield only a three-way meeting in New York that the White House hopes will lay a foundation to resume talks in the future.

At home, Obama's once sky-high approval ratings have tumbled in recent months amid a heated debate over his proposal to reform the healthcare system as well as doubts about his handling of the economy.

EUPHORIA BEGINS TO FADE

Analysts said Obama's weaker domestic popularity probably would not undercut his leadership at the United Nations and G20. But there will be less of the euphoria that marked the summits of his first months in office, including a whirlwind European tour in April where he got a rock-star reception.

We are past this euphoric moment where the president is making his introduction onto the world stage, said Heather Conley, a European affairs expert at the CSIS think tank.

As we transition from this great euphoria, I think we're starting to see a little bit of disquiet set in, Conley said, adding that European leaders were impatient for Obama to turn details into action on climate change and financial reform.

The rancor over healthcare has left in doubt Obama's hopes of passing climate legislation before the Copenhagen global warming summit in December. The bill under consideration falls short of the robust measures Europe had hoped to see.

Europeans, blaming Wall Street recklessness for the crisis that plunged the world into recession, also worry the drive to tighten U.S. financial rules may get sidelined in Congress.

A fight had been looming at the G20 over bonuses for bank executives but leaders have signaled movement toward a compromise. French President Nicolas Sarkozy, who has pushed bonus caps, had threatened to walk out of the G20 over the issue but has since softened his stance.

Obama opposes caps but supports ways to recalibrate pay to remove incentives for recklessness, and the Federal Reserve is close to proposing new rules on the issue.

With the G20 set to consider ways to revive the stalled Doha round of world trade talks, Obama's decision to slap new tariffs on Chinese tires is expected to hamper his ability to lead a call against protectionism and has upset Beijing.

James Lindsay, a former Clinton administration adviser, said Obama would receive a warm reception from foreign leaders but reality is setting in.

Expectations for what President Obama was going to deliver were far too high, he added. The reality of the presidency is that the moment he starts to make decisions, he starts alienating people.

There's always going to be disappointment because countries have read into Obama all of their hopes and dreams, said Lindsay, who is now at the Council on Foreign Relations.

BREAKTHROUGHS UNLIKELY ON MIDEAST

Obama is to sit down on Tuesday with Israeli Prime Minister Benjamin Netanyahu and Palestinian President Mahmoud Abbas but no breakthroughs are expected.

Obama's agenda in New York includes one-on-one talks with Russian President Dmitry Medvedev, Chinese President Hu Jintao and new Japanese Prime Minister Yukio Hatoyama.

A top focus at the United Nations will be preparations for talks planned for October 1 between Iran and other world powers, including the United States.

Obama vowed during his campaign to pursue broader engagement abroad, including with Tehran, but unrest in Iran after its disputed June election has complicated that pledge.

Obama has sought Russian and Chinese support for ratcheting up pressure on Iran over its nuclear enrichment program.

In a move Obama insisted was not aimed at placating Russia, he has said he will shelve a Bush-era missile defense plan for Eastern Europe and replace it with a new version. Obama has faced criticism at home from Republicans who accuse him of weakening Western security and making a concession to Russia.

Obama on Thursday will become the first U.S. president to chair a U.N. Security Council summit. He will focus on nuclear non-proliferation.

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