US President Barack Obama has said the world must tackle stark¹ challenges, and the US cannot face them alone.

美国总统奥巴马称，世界必须一起处理各种挑战，美国不能独立作战。

In his first speech to the UN General Assembly, he said global problems included nuclear proliferation（增殖，分芽繁殖）, war, climate change and economic crisis.

All nations bore responsibility for addressing these problems, he said.

Later, in a long speech, Libyan leader Muammar Gaddafi berated² the UN Security Council, while French President Nicolas Sarkozy issued a stern（严厉的，坚决的） warning to Iran.

Iranian President Mahmoud Ahmadinejad, who has previously³ said he does not believe the Holocaust⁴ happened, was due to speak later on Wednesday.

Israel has called for a boycott⁵（抵制，拒绝参加） of his appearance and the Germans have said they will walk out if he repeats the claim.

'Act together'

In his maiden⁶ speech to the forum⁷ of leaders from more than 120 nations, Mr Obama acknowledged the expectations that accompanied his presidency⁸ - expectations, he said, which were "not about me".

He said that when he took office, "many around the world had come to view America with scepticism（怀疑） and mistrust".

He said some of this was based on "misperceptions（错觉）" but it was also due to "opposition⁹ to specific policies".

But Mr Obama said "no world order which elevates（举起，提拔） one nation above others" could succeed in tackling the world's problems.

"Those who used to chastise¹⁰（笞责，惩罚） America for acting¹¹ alone in the world cannot now stand by and wait for America to solve the world's problems alone," he said.

"We must build new coalitions¹² that bridge old divides... All nations have rights and responsibilities - that's the bargain that makes this work."

The president devoted¹³ a significant proportion of his speech to the challenges of finding a peaceful settlement in the Middle East - and called for the relaunch of "negotiations¹⁴ without preconditions（先决条件）".

The BBC's diplomatic correspondent, Jonathan Marcus, says Mr Obama used his first speech to the General Assembly to signal that the United States was back as a team player on the international stage.

His speech received warm but not effusive¹⁵（流出的，感情奔放的） applause, a sign perhaps that in the face of real world problems the expectations surrounding the president are gradually being adjusted to reality, our correspondent says.

In other contributions to the General Assembly:

UK Prime Minister Gordon Brown said he was looking into reducing the UK's fleet of nuclear weapon submarines from four to three, as part of a "grand global bargain" he proposed between nuclear and non-nuclear states to combat nuclear proliferation

Mr Sarkozy told Iranian leaders they were "making a tragic¹⁶ mistake" if they thought the international community would not respond to what he alleged¹⁷ was their military nuclear programme

UN Secretary General Ban Ki-moon opened Wednesday's proceedings¹⁸ by telling the assembled leaders: "Now is the time to put the 'united' back into the United Nations".