US President Barack Obama on Wednesday sternly enjoined global leaders to work together to solve the world's most pressing problems in his maiden speech to the UN General Assembly.

"Those who used to chastise America for acting alone in the world cannot now stand by and wait for America to solve the world's problems alone," Obama said after receiving warm applause from some 120 heads of state and government.

"Because the time has come for the world to move in a new direction," he said. "We must embrace a new era of engagement based on mutual interest and mutual respect, and our work must begin now."

And in his first address to the 192-member General Assembly, watched intently by Iranian President Mahmoud Ahmadinejad, Obama warned North Korea and Iran had put the world on a "dangerous slope" with their nuclear ambitions.

Libyan leader Moamer Kadhafi stepped up to the podium directly after Obama, although the two men did not meet as Kadhafi delayed proceedings by gathering his notes at his seat.

Clad in sand-colored robes adorned with a black pin in the shape of Africa, a regal-looking Kadhafi spoke for over 90 minutes, breaking strict General Assembly protocol on speech length in his first address to the UN during his 40-year rule.

Long a bugbear of the West, Kadhafi railed against the supremacy of the five veto-wielding permanent members of the UN Security Council ? Britain, China, France, Russia and the United States.

He also accused the major powers of sparking many conflicts since 1945 in pursuit of selfish interests, and demanded that the West pay $7.77-trillion in compensation for colonising Africa.

But other leaders returned to the charge against Iran, as chief diplomats from five nations prepared to gather later to discuss imposing further sanctions on the Islamic republic.

In his address to the assembly, French President Nicolas Sarkozy warned Tehran against "making a tragic mistake" in pressing on with their suspect nuclear activities.

"I would like to say in a solemn manner to the Iranian leaders, that if they rely on a passive response from the international community in order to pursue their military nuclear programme, they will be making a tragic mistake," Sarkozy said.

Protestors also gathered on the streets outside to demonstrate against both the Libyan and Iranian leaders.

"Hey, hey, ho, ho, Kadhafi must go!" a 50-strong group of protestors, including Libyan exiles, chanted at the gates of the UN building.

And more than 500 Iranian exiles also gathered, brandishing placards that called Ahmadinejad a "dictator," waving pre-revolutionary flags, and chanting in front of posters depicting blood-drenched victims of political violence.

Iranian officials have said that Ahmadinejad, while known for his fiery and provocative rhetoric, was to send a message of peace to the assembly ? many of whom fear Iran's nuclear programme has military aims despite Tehran's denials.

Obama called on leaders here to hold Iran ? as well as North Korea ? to account for their nuclear programmes.

"If the governments of Iran and North Korea choose to ignore international standards, if they put the pursuit of nuclear weapons ahead of regional stability and the security and opportunity of their own people... then they must be held accountable," he said.

A day after a landmark climate summit, UN chief Ban Ki-moon opened the debate with a call for "genuine" collective action to roll back climate change and global poverty, and push for nuclear disarmament.

"If ever there were a time to act in a spirit of renewed multilateralism ? a moment to create a United Nations of genuine collective action ? it is now," Ban said.

All eyes were on Obama, who has vowed a close partnership with the United Nations after the administration of his predecessor, George W. Bush, was accused of riding roughshod over the world body.

But the US leader had some sharp words for the United Nations, saying "sadly, but not surprisingly, this body has often become a forum for sowing discord instead of forging common ground."

Israel has called for a symbolic protest against Ahmadinejad, suggesting representatives leave the chamber when the Iranian leader delivers his speech, but it was not yet clear whether many countries would comply or not.

But Canadian Prime Minister Stephen Harper said on Wednesday his government would boycott the Iranian leader's UN speech over his "disgraceful" remarks denying the Holocaust.