Russia expressed alarm on Friday that no date or venue had been agreed for a new round of talks between global powers and Iran over Tehran's nuclear program.
Iran, which denies Western accusations it is seeking to develop a capability to make nuclear weapons, said last week it had agreed to resume talks in January with six 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, which represents the powers, said last week that it had proposed a date to Iran but Western diplomatic sources said on Friday that Iran 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, he source said.
Global powers, particularly in the West, want to rein in Iran's uranium enrichment work. Tehran says it is refining uranium for peaceful ends only but enrichment yields material that can be used to make nuclear bombs if processed further.
There was no breakthrough in three rounds of talks last year, the most recent in Moscow in June, and Israel has stepped up talk of pre-emptive attacks on Iranian nuclear sites, lending urgency to diplomacy.
Ryabkov said he hoped the talks will take place this month.
"When we parted in June after the Moscow round, we agreed that the process should continue without substantial breaks," Interfax quoted him as saying.
Western diplomats and analysts say it is unclear why Iran has not agreed a date for new talks. Some suggest Tehran may want to wait until after its next meeting with the U.N.'s International Atomic Energy Agency, next Wednesday.
IAEA chief Yukiya Amano said in Tokyo on Friday he was not optimistic about the talks or getting access to a military base Western powers suspect has been used for atom bomb-related work.
"The outlook is not bright," Amano said.
He was referring to negotiations to be held in Tehran on the framework accord the Vienna-based IAEA hopes will enable it to quickly resume its stalled investigation into suspected atom bomb research.
His remarks contrasted with a more upbeat assessment given by the IAEA after a meeting with Iranian officials last month.
"Talks with Iran don't proceed in a linear way," 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, which built Iran's first nuclear power plant, has supported four rounds of U.N. Security Council sanctions against Tehran over its nuclear program but opposes further measures and has sharply criticised Western sanctions.
Russia has adamantly warned against attacking Iran and, while it says Tehran must cooperate and dispel concerns about its nuclear program, officials including Ryabkov have suggested Western fears it seeks nuclear weapons are overblown.