MOSCOW/VIENNA (REUTERS) - Russia voiced alarm on Friday at delays in agreeing new nuclear talks between world powers and Iran and the United Nations (UN) atomic watchdog chief said he was not optimistic ahead of his inspectors' separate visit to Teheran next week.

The comments underlined the difficult challenges facing world powers in their search for a diplomatic solution to the decade-old standoff over Iran's nuclear programme to avert the threat of a new Middle East war.

Iran, which denies Western accusations it is seeking to develop a capability to make nuclear weapons but is facing intensifying sanctions pressure, said last week it had agreed to resume negotiations in January with the six major powers.

But Russian Deputy Foreign Minister Sergei Ryabkov said on Friday there was no final agreement on when or where a meeting would take place.

"This alarms us, because the pause has dragged on," the Interfax news agency quoted Ryabkov, who is the Russian negotiator, as saying. "As a nation and a member of the 'group of six', we are working actively to find a solution."

The European Union (EU), which represents the powers, said last week that it had proposed a date to Iran but Western diplomatic sources said on Friday that Teheran had yet to respond.

One source suggested that the date the EU proposed was next Tuesday but said that was now unlikely.

"It is our understanding that Iran has not responded to the Jan. 15 date," the diplomatic source said.

The six powers - the United States, France, Britain, China, Russia and Germany - were therefore not planning for that.

Ryabkov said he hoped the talks will take place this month.

A former Iranian nuclear negotiator said Iran was ready to meet "as soon as possible", but suggested that uncertainty in Teheran about what the other side would offer may make it reluctant to agree a new date now.

The fact that the new administration of US President Barack Obama - including new defence and foreign secretaries - is not yet up and running and in a position to offer Iran a"fair package" may be another factor, Mr Hossein Mousavian said.

"OUTLOOK NOT BRIGHT"

The powers want Iran to scale back its uranium enrichment programme and cooperate fully with the UN nuclear watchdog.

Iran, one of the world's largest oil producers, wants the West to lift sanctions hurting its economy.

There was no breakthrough in three meetings last year, the most recent in June, as Iran rejected a proposal to halt its higher-grade enrichment and close an underground nuclear site in exchange for reactor fuel and civil aviation parts.

If the powers "present a package a little bit modified then again the talks would fail and they would blame Iran", Mr Mousavian, now a visiting scholar at Princeton University, told Reuters.

Some Western diplomats suggest Teheran may want to wait to set a date for the talks with the powers until after its next meeting with the watchdog International Atomic Energy Agency (IAEA) in Teheran next Wednesday.

The talks between the IAEA and Iran are separate from, but linked to, broader diplomacy by six world powers to resolve the nuclear row with Iran before it degenerates into war, feared because of Israeli threats to bomb Iranian nuclear sites.

IAEA chief Yukiya Amano said in Tokyo on Friday he was not optimistic about the talks on getting access to a military base Western powers suspect has been used for atom bomb-related work.

"The outlook is not bright," Mr Amano said.

The IAEA has been trying for a year to reach a framework accord with Teheran that would enable the agency to resume its stalled investigation into suspected nuclear weapon research by the Islamic Republic.

"Talks with Iran don't proceed in a linear way," Mr Amano said in Japanese comments translated into English. "It's one step forward, two or three steps back... So we can't say we have an optimistic outlook" for the Jan 16 meeting.

Russia has adamantly warned against attacking Iran and, while it says Teheran must cooperate and dispel concerns about its nuclear programme, officials including Mr Ryabkov have suggested Western fears it seeks nuclear arms are overblown.