Iran has promised to allow United Nations nuclear officials better monitoring and access to a site where it started enriching uranium to higher levels over two months ago, diplomats said.

The move should have happened earlier, diplomats said, as Iran started higher enrichment in February, before inspectors from the International Atomic Energy Agency (IAEA) could improve surveillance.

IAEA spokeswoman Gill Tudor confirmed Saturday that the Director General Yukiya Amano is scheduled to meet Iran?s Foreign Minister Manouchehr Mottaki on Sunday, Iranian news agency IRNA reported.

According to the report, Mottaki, who will be on a planned visit to Vienna, is due to meet the IAEA director general to discuss issues of mutual interest, as well as Austrian officials.

Suspecting that Iran aims to develop atomic weapon, Western powers have said the switch to higher enrichment was "provocative" and warranted sanctions.

In meetings with IAEA officials earlier this month, Tehran agreed "in principle" to allow them to strengthen monitoring and containment measures to improve chances of detecting any diversion of nuclear material for military use, diplomats said.

"Now an agreement has been reached for enhanced surveillance measures. But we have to see if and when they are put in place," a senior Western diplomat said.

The Islamic Republic said its promise showed it was cooperating well with the IAEA.

Western diplomats welcomed the concession, but said that these were routine steps that Tehran had shirked.

Iran says it is enriching uranium to 20 percent purity to create material for fabrication into fuel for a medical research reactor.

This has drawn suspicion abroad, as Tehran is thought to lack the technology to complete such a plan soon.

Since the work started, the IAEA has sought to improve camera surveillance angles, add cameras, conduct more frequent visits and inspections at short notice at the site and to add equipment to measure enrichment levels, diplomats said.

The UN watchdog had wanted these measures in place before the start of higher-grade enrichment at the site three months ago.

Expansion concerns

Western officials fear Iran's move to escalate enrichment is ultimately meant to advance it on the road to generating weapons-grade uranium -- enriched to 90 percent purity. Iran says its nuclear work is for peaceful purposes only.

"In principle, yes, we have agreed to make sure that the legitimate, technically-justified requests of the IAEA are fulfilled," said Ali Asghar Soltanieh, Iran's envoy to the IAEA.

"The enhanced measures are also important in case Iran wants to start enriching on more than one cascade," a diplomat close to the IAEA said, referring to a unit of 164 centrifuge enrichment machines.

An IAEA report in February said Iran had produced a small amount of higher-grade enriched uranium on one cascade at Natanz. Iran has said one cascade is enough to produce the material it needs -- about 1.5 kg a month.

But it would make sense for Iran to expand enrichment to more cascades to make the process more efficient if its ultimate aim is to make nuclear fuel, analysts say. An expansion would further escalate tensions with Western powers.

"If they are serious about wanting to produce 20 percent enriched uranium they would have to expand. It is very wasteful otherwise," said Mark Fitzpatrick at London's International Institute for Strategic Studies.

Iran said it was forced to enrich to higher levels after the breakdown of a fuel swap deal with Western powers and IAEA, under which it would have sent much of its low-enriched uranium abroad in return for fuel rods for the medical reactor.

Fitzpatrick said Iran was both demonstrating defiance and seeking to pressure the West into accepting its counterproposal for a fuel deal.

Iran wanted changes to the fuel offer, such as a simultaneous swap on Iranian soil, that other parties find unacceptable.

Diplomats said Tehran was lobbying to avoid new sanctions. Iranian Foreign Minister Mottaki will be in Vienna, where the IAEA is headquartered, on Sunday.