UN atomic watchdog chief Yukiya Amano expressed "great regret" Monday at Iran's decision to bar key inspectors from the country, saying it hampered the agency's investigation of Tehran's nuclear programme.

"I learned with great regret about Iran's decision to object to the designation of two inspectors who recently conducted inspections in Iran," Amano told the 35-member board of the International Atomic Energy Agency.

"Iran's repeated objection to the designation of inspectors with experience in Iran's nuclear fuel cycle and facilities hampers the inspection process," he said, according to a copy of his speech.

"If this continues unchecked, it will be problematic," Amano warned at a news conference afterwards.

In a restricted report circulated to board members last week, Amano had already complained about Tehran's so-called "de-designation" of inspectors, particularly in the recent cases of two experienced inspectors who had their permits revoked after Iran alleged they had made "false" reports.

But Amano stood by his staff on Monday.

"I express my full confidence in the professionalism and impartiality of the inspectors concerned," he said. "Both are very knowledgeable about the nuclear fuel cycle and have long experience in Iran."

Tehran's envoy to the IAEA, Ali Asghar Soltanieh, rejected the suggestion that the barring of two inspectors could hamper the agency's work.

"I categorically reject this statement," Soltanieh told reporters.

"They're trying to make an issue (out of this)," he said, insisting it was Iran's right, under the terms of its safeguards agreement with IAEA, to vet inspectors. Furthermore, member states were not obliged to provide a reason for such a decision.

It was "ridiculous" for the agency to complain about the decision to bar just two inspectors when there was a pool of "over 150 inspectors" to draw from, Soltanieh said.

Washington's ambassador to the IAEA, Glyn Davies, suggested Tehran was trying to intimidate inspectors from asking too many awkward questions.

"It sends a chill through the ranks of the inspectors," Davies said. It could be seen as a signal to inspectors that "if they report what they see, and if that ends up in a director general's report, they might be taken out and shot at dawn, metaphorically speaking," the US ambassador said.

Traditionally, the September board meeting prepares for the IAEA's annual general conference -- which brings together all 151 member states -- next week.

And in past years, the general conference has been dominated by bitter debates between Arab states on the one hand and western states on the other over Israel's presumed nuclear arsenal.

Last year, the Arab states secured narrow backing for a resolution calling on the Jewish state -- widely believed to be the only power in the Middle East with nuclear weapons -- to join the nuclear Non-Proliferation Treaty.

Washington's aim was to persuade Arab states not to forward their text again this year, because singling out Israel would be divisive, Davies said.

And it could jeopardise the agreement reached in New York in May for a regional conference in 2012 to advance the goal of a nuclear-free Middle East, he said.

On Monday, agency chief Amano named four new deputy directors general. And for the all-important position of head of safeguards, effectively the IAEA's top investigator, Amano picked Belgian-born Herman Nackaerts, 59, to take over from Olli Heinonen, who resigned at the end of August.

The safeguards department is responsible for verifying that countries' nuclear activities are exclusively peaceful and not diverted for military purposes.

Heinonen, 63, stepped down after five years on the job and nearly 30 years in the agency.

Nackaerts joined the IAEA in 2006, when he was appointed Heinonen's deputy. Prior to that, he was head of safeguards inspections at the European Commission.