US President Barack Obama told the UN General Assembly on Tuesday that there should be a basis for an agreement on Iran's nuclear ambitions but that the roadblocks will be difficult to overcome.

In an address laying out US policy toward the volatile Middle East and North Africa, Obama made clear that the United States will take direct action to eliminate threats when necessary and will use military force when diplomacy fails.

Obama, in closely watched remarks on Iran based on a diplomatic opening offered by Iran's new president, Hassan Rouhani, said the US wants to resolve the Iran nuclear issue peacefully but is determined to prevent Iran from developing a nuclear weapon.

"The roadblocks may prove to be too great but I firmly believe the diplomatic path must be tested," Obama said.

He urged the UN Security Council to approve a strong resolution aimed at ensuring Syria keeps its commitments to give up chemical weapons and said the US will provide an additional $340 million in humanitarian aid.

The president's address is be closely watched for signs that he may meet later in the day with Rouhani, a moderate cleric who has been making friendly gestures toward the US in recent weeks.

A meeting between Iran's top diplomats and world powers at the UN General Assembly will start a "new era" in efforts to end the dispute with the West over Teheran's nuclear program, the Iranian Foreign Ministry said on Tuesday.

It did not hint at any concessions by Teheran.

Former Chinese ambassador to Iran Hua Liming said that there are high hopes of a breakthrough in relations between Washington and Teheran on the sidelines of the UN assembly.

"We have noticed that Iranian President Rouhani has followed relatively mild foreign policies," Hua said.

"Recent positive exchanges between Rouhani and Obama paint at least an effort at moderation in Iran's foreign policy," Xinhua News Agency said in a commentary on Tuesday.

The European Union said on Monday that Iranian Foreign Minister Javad Zarif would join a meeting of major powers - including Britain, France, China, Russia, the US and Germany - to discuss the Iranian nuclear program.

The meeting, due on Thursday and expected to include US Secretary of State John Kerry, would be the highest-level encounter involving the two nations since relations were severed in 1980 at the height of the US embassy hostage crisis.

According to Mehr News Agency, Iranian Foreign Ministry spokeswoman Marzieh Afkham told a news conference: "These talks are the start of a new era. The Islamic Republic has explicitly stated its views regarding its rights to peaceful nuclear energy and the right to enrich (uranium) on Iranian territory.

"This meeting represents a serious commitment of the foreign parties to reach a solution based on a specified time-frame," she said.

Before leaving for New York on Monday, newly elected President Rouhani said he wanted to present Iran's "true face" and to pursue talks and cooperation with the West to end the nuclear dispute.

Iran says it is enriching uranium only to fuel a planned network of nuclear power stations and for medical purposes. Rouhani's election as Iranian president in June raised hopes of a more conciliatory approach from Iran in seeking to diffuse the ongoing nuclear issue.

The US and its allies have imposed tough economic sanctions on Iran over suspicions that it is seeking a nuclear weapons capability.

US officials have also said a meeting is possible this week between Obama and Rouhani, a landmark after more than three decades of hostility.

AP-Reuters-China Daily

Li Xiaokun in Beijing contributed to this story.