The UN atomic agency's board of governors began meeting in Vienna on Monday with Iran's nuclear drive topping the agenda as the IAEA seeks access to a suspect Iranian military base.

The 35-member board meeting of the International Atomic Energy Agency (IAEA), which is being held behind closed doors all week, comes amid a flurry of international efforts to curtail Tehran's nuclear activities.

In its latest report issued last month, the nuclear watchdog said new satellite imagery of the Parchin military base near Tehran indicated "extensive activities" where there had been "virtually" none for years.

This included the razing of two buildings and what experts saw as signs of a clean-up at the base, where the nuclear watchdgo believes suspicious explosives testing was carried out.

This "could hamper the agency's ability to undertake effective verification" of the site, the IAEA report warned.

The agency previously sought access to the base during two visits in January and February but was denied by Tehran, which insists Parchin is not linked to its nuclear programme so it is not obliged to allow inspections.

Iran insists its atomic programme is solely for civilian purposes but Western powers believe it is trying to develop a bomb, and Israel and the United States have not ruled out possible military action.

After a visit to Tehran on May 21, IAEA chief Yukiya Amano said he and Iran's chief nuclear negotiator Saeed Jalili would soon sign a deal to resolve issues on the nuclear programme.

But two weeks on, there is still no sign of any agreement.

Last week, IAEA chief inspector Herman Nackaerts also showed delegations new satellite pictures of Parchin taken on May 25 that analysts say suggest cleanup activities at the military base.

Iran's enrichment of uranium to 20-percent purity, bringing Tehran consistently closer to producing 90-percent enriched uranium needed to make a bomb, is another matter of concern, according to Western powers.

Iran and the P5+1 powers -- the United States, Russia, China, Britain, France and Germany -- met in Baghdad on May 23-24 in a bid to ease tensions but little was achieved.

A further meeting is due to be held in Moscow on June 18-19, before an EU oil embargo against Iran comes into force on July 1.

On Sunday, Iran's supreme leader Ayatollah Ali Khamenei said claims that Tehran was seeking nuclear weapons was "based on a lie" and insisted that sanctions on his country were ineffective and only strengthened its resolve.

He also warned arch-enemy Israel that any attack on Iran "will fall on their heads like thunder".