Iran came under mounting pressure for its nuclear defiance as Russia signalled it could back biting sanctions if Tehran fails to make concessions in crunch talks next month.

In a busy day of diplomatic meetings on the sidelines of the UN General Assembly, Russia appeared to make a policy shift on the issue of slapping more sanctions on the Islamic republic.

"Russia's position is simple: sanctions are seldom productive but they are sometimes inevitable. It is a matter of choice," Russian President Dmitry Medvedev said after talks with US President Barack Obama.

In his eagerly-awaited maiden speech to the 192-member assembly, watched intently by Iranian President Mahmoud Ahmadinejad, Obama warned that Tehran and Pyongyang's nuclear push had put the world on a "dangerous slope."

"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 told some 120 world leaders.

Iran's suspect nuclear ambitions were increasingly under fire throughout the day as world leaders warned it to halt its uranium enrichment programme.

"Serious additional sanctions remain a possibility," Obama said after conferring with Medvedev, urging Tehran to "seize the opportunity" at key talks with six major powers scheduled for 1 October in Geneva.

British Prime Minister Gordon Brown delivered a similar message, telling the assembly that both Iran and North Korea must comply with UN demands.

"Iran and North Korea must now know that the world will be even tougher on proliferation. We are ready to consider further sanctions," he said.

The warnings came a day before Obama was to chair an unprecedented summit on nuclear proliferation in the UN Security Council.

French President Nicolas Sarkozy also warned Tehran against "making a tragic mistake" in pressing on with their suspect nuclear activities.

And there was tough talk too from chief diplomats of the six major powers ? Britain, China, France, Germany, Russia and the United States ? which have been trying to clip Tehran's nuclear ambitions.

The West suspects the Islamic republic is trying to acquire a nuclear weapons capability under the cover of his civilian atomic programme.

But Tehran denies the charge and maintains that it will instead discuss in Geneva proposals to promote global nuclear disarmament.

Meanwhile, a speech to the assembly by Ahmadinejad triggered a walkout by a dozen delegations, including the United States and France.

"It is disappointing that Mr Ahmadinejad has once again chosen to espouse hateful, offensive and anti-Semitic rhetoric," Mark Kornblau, spokesperson to the US mission to the United Nations, said in a statement.

Delegations from Argentina, Australia, Britain, Costa Rica, Denmark, France Germany, Hungary, Italy, New Zealand and the United States left the room as Ahmadinejad began to rail against Israel, a European source said.

Ahmadinejad again took aim at Israel without mentioning the country or Jews by name, referring only to the "Zionist regime".

The firebrand leader, re-elected in disputed June elections, accused Israel of "inhumane policies in Palestine" and suggested it was behind "a genocide" in the occupied Palestinian territories.

Earlier in the day, Obama had used his address to challenge global leaders to work with his administration to solve the world's most pressing problems.

"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.

Libyan leader Moamer Kadhafi stepped up to the podium directly after Obama, although the two men did not meet.

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 protocol in his first address to the UN during his 40-year rule.

Kadhafi berated Western powers in a UN diatribe, accusing the global body of failing to prevent millions of deaths as he demanded trillions of dollars in colonial reparations.

The maverick Libyan gave what may go down among the more memorable speeches to the UN General Assembly, referencing issues as diverse as the John F. Kennedy assassination, Philippine emigration and swine flu as many fellow leaders gradually drifted out of the room.

Protestors meanwhile 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.