MOSCOW (Reuters) - Russia on Tuesday said a new round of talks between Iran and world powers over its nuclear program must no longer be delayed and should take place by mid-September.

World powers hope that Iran's relatively moderate new President Hassan Rouhani, a cleric with links to all of Tehran's often-feuding factions, can defuse tension over a nuclear program that they suspect is aimed at making bombs. Iran says its nuclear activities are purely for civilian purposes.

Deputy Foreign Minister Sergei Ryabkov said Moscow shared the optimism of the five other world powers involved in nuclear talks with Iran and will push to organise a new round of talks as soon as Rouhani names his nuclear negotiator.

"This round, given all the circumstances, must be held by mid-September. This cannot be delayed any longer," Ryabkov told the Interfax news agency.

"As soon as the configuration of the new Iranian delegation is clear, the group of six can quickly make a proposal and organise for this round in the next 4-5 weeks," he said. "There is a lot of reason for optimism."

Rouhani's choice for nuclear negotiator will help determine the tone and tactics of Iran's diplomacy with world powers.

(Reporting by Alissa de Carbonnel; Editing by Sonya Hepinstall)