Senior U.N. nuclear officials are set to travel to Tehran later this month for a rare visit to discuss their growing concerns that Iran may be seeking to develop atomic arms capability, diplomatic sources said on Thursday.

A high-level team from the U.N. International Atomic Energy Agency (IAEA) is expected to visit Iran around January 28, even though the exact timing has not yet been finalised, two sources said, one of them suggesting it could also happen a day later.

Such a trip would come at a time of escalating tension over Iran's nuclear ambitions with European nations preparing for an embargo on Iranian oil and Tehran threatening to retaliate by blocking Gulf oil shipping lanes vital to the global economy.

Iran, which has stoked Western suspicions by starting to enrich uranium inside a mountain bunker, last month said it had renewed an invitation for a special IAEA team to visit the country.

Earlier this week, an IAEA official told Reuters that the team, to include Deputy Director General Herman Nackaerts and other senior officials, would travel to Tehran quite soon.

Iran's envoy to the Vienna-based U.N. agency has said the talks would be aimed at resolving the issue - a reference to the long-running nuclear dispute - and suggested Iranian officials would be ready to address the IAEA's concerns.

But some Western diplomats expressed scepticism. I doubt very seriously it will lead to anything, one diplomat said.

The IAEA has often complained that Tehran has failed to engage in substance regarding suspicions that it may be working to develop the means needed to make nuclear bombs.

The Islamic Republic has also signalled readiness to resume talks with major powers that have been frozen for a year.

Western diplomats tend to see such initiatives as attempts by Iran, a major oil producer, to buy time for its nuclear programme, without heeding U.N. demands to curb activity that could be put to making atomic bombs.

Iran has come under increased pressure since the IAEA reported in November that Tehran appeared to have worked on designing a nuclear weapon and that secret research to that end may be continuing, charges the country denies.

Iran says its nuclear programme is entirely peaceful, aimed at generating electricity and producing medical isotopes to treat cancer patients.

(Reporting by Fredrik Dahl)