The UN atomic watchdog will give details next month on what it suspects may be covert Iranian efforts to develop nuclear weapons, sources said Friday, in a move set to further stoke tensions.

The new assessment, due to go before the board of the International Atomic Energy Agency (IAEA) on November 17-18, according to a provisional agenda seen by AFP, "should be more complete," one diplomat to the watchdog said.

The US envoy to the IAEA, Glyn Davies, meanwhile said on Thursday in Santiago, Chile, that he hoped the agency would provide a "sharpening of the case."

In comments confirmed by his office, Davies said however that it was so far unclear whether this would prompt the agency's 35-member board to report Iran again to the UN Security Council.

"We expect the IAEA to begin to get more explicitly into the issue of ... the possible military dimensions," Davies said. "We'll see whether there's enough there for further action by the board."

Such a referral to the Security Council, which has already imposed four rounds of sanctions on Iran, would represent a further increase in pressure on the Islamic republic, which insists its nuclear activities are peaceful.

Tensions between Washington and Tehran rose further this week after the United States accused Iran of being behind an alleged plot to assassinate the Saudi Arabian ambassador on US soil. Iran has angrily denied any involvement.

IAEA director general Yukiya Amano said in a September report that he was "increasingly concerned" about the "possible military dimension" of Iran's nuclear programme.

The Vienna-based agency had said its information was "extensive and comprehensive and has been acquired both from many (IAEA) member states and through its own efforts."

It added it was more and more worried by "the possible existence in Iran of past or current undisclosed nuclear-related activities involving military-related organisations, including activities related to the development of a nuclear payload for a missile."

In another report in May, Amano listed seven areas of concern including testing high explosives, studies of detonators and design work on arming missiles with a nuclear payload.

Iran's nuclear programme has been slowed by an attack by the Stuxnet computer virus, the assassination of nuclear scientists and by the UN sanctions, experts say.

Analysts are divided over how far Iran is from being able to have a nuclear weapon, but most agree that it is making progress on all areas that would allow it to do so, including enriching uranium and in weapons development.

Tehran plans to increase its uranium-enrichment activities and has started fitting out a difficult-to-bomb new mountain bunker site at Fordo, the IAEA says.

Last year, Iran began enriching uranium to a purity of around 20 percent, taking it closer to the 90-percent level needed for a nuclear weapon.

Analysts differ on how long it would take Iran to enrich enough uranium to 90 percent to make a bomb, following a decision to do so.

Greg Jones of the US-based Nonproliferation Policy Education Center said last month this would take just eight weeks, and that Iran was making such progress that by the end of 2012, this would be cut to four weeks.

Others, though, are more cautious.

Mark Fitzpatrick, for example, director of the Non-Proliferation and Disarmament Programme at the London-based International Institute for Strategic Studies (IISS), said earlier this month it would take much longer.

Enriching to 90 percent could provoke air strikes, the many steps needed to create a weapon would take at least six months, and in the end they might only produce one bomb -- which might not even work, he says.

"What country would take all of the risks of breaking out of the NPT (nuclear non-proliferation treaty), including the likelihood of inviting airstrikes, to produce one weapon?"