* New phase of nuclear diplomacy with Iran to begin in February
* Obama warns of challenging talks
* Interim nuclear deal goes into effect on Jan. 20 (Adds Russian comments, context, details)
By Justyna Pawlak
BRUSSELS, Jan 13 (Reuters) - World powers and Iran will very likely meet again on Tehran's nuclear ambitions in February, shortly after a six-month deal restricting its atomic work goes into effect, a diplomatic source told Reuters on Monday.
The new round of talks will aim for a broad settlement in the decade-old dispute over the Islamic Republic's nuclear programme that could ease the risk of mistrust leading to deeper tensions or even conflict in the Middle East.
The talks, led by the EU's foreign policy chief Catherine Ashton, will face the challenge of defining a permissible scope of Iranian nuclear work that will lay to rest western concerns that it could have military goals.
In return, Iran wants governments in the United States and Europe to end decades of painful economic sanctions.
The source said the first meeting in the new phase of diplomacy between Iran and six powers - the United States, Russia, China, France, Britain and Germany - would include Ashton and Iranian Foreign Minister Mohammad Javad Zarif.
"It won't happen in January, because of the Chinese New Year, but it is very, very, very likely in February," the diplomat said, speaking on condition of anonymity.
Senior diplomats from the seven countries will gather ahead of the meeting to prepare an agenda.
Iran says its atomic energy programme is aimed purely at electricity generation and other civilian purposes, although past Iranian attempts to hide sensitive nuclear activity from U.N. non-proliferation inspectors have raised concerns.
The interim six-month agreement, which freezes Iran's most sensitive atom work in return for some sanctions relief, is scheduled to go into effect on Jan. 20, pending verification by the International Atomic Energy Agency that Tehran is meeting its end of the deal.
That accord appeared to halt a slide towards another, wider Middle East war over Iran's nuclear aspirations, but diplomats warn it will not be easy to carry out because of long-standing mutual mistrust.
Partly in preparation for new talks, Iranian Foreign Minister Zarif will meet Russian President Vladimir Putin in Moscow this week, Russian officials said on Monday.
"During his one-day trip to Moscow, Zarif will discuss bilateral and regional issues with top Russian officials. Also some topics related to Iran's nuclear programme will be discussed," the Iranian state news agency IRNA quoted Iran's ambassador to Russia, Mehdi Sanai, as saying.
Russia's Foreign Ministry issued a statement on Monday welcoming a weekend decision by the six powers and Iran to set Jan. 20 as the date to implement the interim accord.
"We hope that a successful implementation of the primary phase will create the necessary conditions for working out agreements going further that will result in a final and comprehensive settlement in regard to the Iran's nuclear programme," it said.
Russia and Iran are also negotiating an oil-for-goods swap worth $1.5 billion a month, sources have told Reuters.
Russian and Iranian sources close to the barter negotiations have said final details were being discussed for a deal under which Russia would buy up to 500,000 barrels a day of Iranian oil in exchange for Russian equipment and goods.
U.S. President Barack Obama said on Sunday that he had "no illusions about how hard it will be" to get a comprehensive agreement with Iran but that it was vital "for the sake of our national security and the peace and security of the world" to try to find a diplomatic solution.
The United States and Israel have both refused to rule out the possibility of military action against Iran's nuclear programme if the matter cannot be resolved diplomatically. (Additional reporting by Parisa Hafezi in Ankara and Gabriela Baczynska in Moscow, editing by Luke Baker and Tom Heneghan)