Negotiators failed Friday to reach an agreement on whether Iran should be invited to Syria peace talks in Switzerland next month, but Tehran is not yet "off the list", global peace envoy Lakhdar Brahimi said.

"On Iran, we haven't agreed yet. It's no secret that we in the United Nations welcome the participation of Iran, but our partners in the United States are still not convinced that Iran's participation would be the right thing," Brahimi told reporters after talks with US and Russian officials.

"We have agreed that we will be talking a little bit more to see if we can come to an agreement about this," said the veteran Algerian mediator, tasked by the United Nations and the Arab League with brokering peace talks.

With a Syria peace conference finally due to start in Switzerland on January 22, there has been persistent wrangling over a role for key player Iran.

"We find it difficult to imagine them at this conference," a senior US administration official told reporters on condition of anonymity.

Besides lending direct support to Syrian President Bashar al-Assad, Iran bankrolls the Lebanese Shiite Hezbollah fighters bolstering his forces.

Both Brahimi and the US official stressed that Iran could play a role even without officially attending the conference.

"No one has said that Iran can't be consulted by the UN, no one has said that Iran can't take actions that would improve the situation," the official said.

Key Assad ally Russia has sought to have Iran at the table.

Moscow's strong support of Assad was highlighted Thursday when it blocked a US-sponsored UN Security Council statement denouncing his government for its brutal offensive on the northern city of Aleppo, where scores of civilians have been killed in recent missile and "barrel bomb" attacks.

Western nations have pushed for Saudi Arabia to take part, and the Sunni kingdom is on the list of two dozen nations invited to the talks, Brahimi said.

Saudi Arabia and fellow Sunni monarchies in the Gulf -- such as Qatar -- are major backers of the rebels in the war which has morphed into a sectarian conflict between Islam's two main branches.

Brahimi and senior US and Russian officials met behind closed doors at the United Nations in Geneva, then held broader talks with fellow UN Security Council permanent members Britain, China and France.

He then sat down with envoys from Syria's neighbours Iraq, Jordan, Lebanon and Turkey -- who are due to attend the talks, and have taken in the bulk of the 2.4 million refugees from a war that to date has claimed over 126,000 lives.

Brahimi expressed "anger and regret" at the bloodshed, the humanitarian crisis compounded by blockades on aid, arrests for no reason and kidnappings.

"We hope that now we have a date for the conference, the parties will unilaterally take a number of decisions, as measures to indicate they are coming to Geneva to end this conflict," he said.

Who will represent Syria's sides?

Beyond the Iran issue, all eyes are on the potential list of delegates from Syria's warring sides.

"The government has officially informed us that they already have formed their delegation," Brahimi said, adding that Damascus was set to make the delegates public soon.

Moscow's pointman on the Syria crisis, Deputy Foreign Minister Gennady Gatilov, met with Brahimi Friday in Geneva, and was later quoted by Russian news agencies as saying Foreign Minister Walid Muallem would lead Damascus' delegation.

The opposition, meanwhile, is split between the Syrian National Coalition, which backs the conference, and hardliners who say even talking to the Assad regime is a betrayal.

"We met representatives of the coalition and they told us they are reaching out to others, inside and outside of Syria," Brahimi said, with the delegation expected to be formed over coming days.

Washington is meanwhile optimistic that the deeply splintered opposition will manage to pull together a delegation that is "credible, legitimate and as representative as it can be," the US official said.

The Western-backed rebel Free Syrian Army, once the country's strongest armed opposition force but now increasingly marginalised by Islamists, called Friday for unity in the rebel ranks.

Having begun as a rag-tag collection of military defectors and civilians taking up arms to defend peaceful anti-Assad protesters from a March 2011 crackdown, the rebels have been increasingly torn by ideological differences and conflicting interests.

French President Francois Hollande warned that the conference could not be a success if it confirmed Assad in power.

The meeting cannot be "an objective in itself", he said on the sidelines of an EU summit in Brussels.

The so-called Geneva II conference is a follow-up to one held in the Swiss city in June 2012, where world powers called for a Syrian transition government.

But the warring sides failed to agree on whether Assad or his inner circle could play a role in the process, and amid spiralling fighting the plans for Geneva II were repeatedly put on hold.

The multinational January 22 opening session will be held in Montreux, a city northeast of Geneva, before talks involving the opposing Syrian delegations and Brahimi are to continue in Geneva from January 24.

Syria peace talks guest list
Geneva (AFP) Dec 20, 2013 - UN-Arab League envoy Lakhdar Brahimi on Friday announced the list of delegations set to take part in next month's long-awaited Syria peace conference in Switzerland.

Wrangling continues over a potential role for Syria's staunch ally Iran, and the make-up of the Syrian government and rebel delegations is still to be announced, but the remaining players have been settled, Brahimi said.

The following is a list of the organisations and countries due to be involved:

United Nations, with Secretary General Ban Ki-moon as conference chairman

Permanent UN Security Council members Britain, China, France, Russia and the United States

Arab League head Nabil al-Arabi

European Union foreign policy chief Catherine Ashton

Organisation of Islamic Cooperation head Ekmeleddin Ihsanoglu

Algeria

Brazil

Canada

Denmark

Egypt

Germany

India

Indonesia

Iraq

Italy

Japan

Jordan

Kuwait

Lebanon

Morocco

Norway

Oman

Qatar

Saudi Arabia

South Africa

Spain

Sweden

Switzerland

Turkey

United Arab Emirates