An international peace conference aimed at ending Syria's civil war will be held on January 22, the first face-to-face talks between the government of President Bashar al-Assad and rebels seeking to overthrow him, the United Nations said on Monday.

The United Nations is hoping for a peaceful transition in Syria, building on an agreement between world powers reached in June last year. The deal calls for the warring sides to agree to set up a transitional governing body with full executive powers, including over military and security entities.

"We will go to Geneva with a mission of hope," U.N. Secretary-General Ban Ki-moon said in a statement.

The announcement came as Syria mediator Lakhdar Brahimi met senior U.S. and Russian officials in Geneva in his latest effort to get negotiations on track to end a war, now in its third year, that has killed more than 100,000 people.

Brahimi, with backing from world powers, has been trying to convene a peace conference since May and had hoped that it could be held in mid-December. He will hold a news conference later on Monday to announce the invitees, a spokeswoman said.

The participation of Syria's ally Iran in the peace conference has been a major stumbling block as Washington has opposed it, while Russia has backed Tehran's attendance.

It was not clear from Ban's statement whether Iran would be invited. He said he expected "all regional and international partners to demonstrate their meaningful support for constructive negotiations".

Brahimi has previously called for Iran to be included in the conference.

A U.S. official said U.S. Secretary of State John Kerry would attend the talks.

World powers including the United States clinched a deal on curtailing Iran's nuclear program at the weekend, in a sign of easing tensions between the longtime foes.

Brahimi, after separate talks with Russian Foreign Minister Sergei Lavrov and Iranian Foreign Minister Mohammad Javad Zarif at the weekend, met leaders of the Syrian opposition on Sunday.

Factional fighting and fragmentation among those seeking to overthrow Assad have hampered the revolt as well as diplomatic efforts to form a representative opposition party to negotiate with Assad's delegation.

Islamist fighters in Syria joined forces on Friday to form what may be the biggest rebel army in the country, further undermining Western-backed military commanders.

U.N. humanitarian chief Valerie Amos has called a meeting in Geneva on Tuesday on the need for aid workers to have access to deliver supplies to besieged civilians in Syria.

The fate of civilians in the towns of eastern Gouta, the Old City of Homs and Mouadamiya on the edge of Damascus is of utmost concern, Western diplomats say. The Assad government says the residents of Mouadamiya are being "held hostage" by terrorists.