BEIJING, Dec. 16  -- China on Tuesday expressed optimism for the latest Iran nuclear talks which begin in Geneva on Wednesday, vowing to play a constructive role in promoting a comprehensive nuclear agreement.

Wang Qun, director-general of the Chinese Foreign Ministry department of arms control, will head the Chinese delegation to the talks, said Foreign Ministry spokesman Qin Gang at a regular press briefing.

Iran and the world powers have agreed to extend the talks until June 30, 2015, giving more time and room for maneuver, Qin said.

He urged parties concerned preserve the momentum of negotiations, find a consensus in a step-by-step and reciprocal way, striving for a mutual-beneficial comprehensive nuclear deal as soon as possible.

After the deal between Iran and the P5+1 group on Nov. 24, 2013, several rounds of talks have failed to work out a comprehensive agreement. Under the interim deal, Iran will suspend some sensitive nuclear activities in exchange for limited relief of sanctions.