Iran's nuclear work will defiantly go on, supreme leader Ayatollah Ali Khamenei said today, after UN inspectors left Tehran following talks that failed to lift their suspicions of atomic weapons research.

Iran's nuclear work will defiantly go on, supreme leader Ayatollah Ali Khamenei said today, after UN inspectors left Tehran following talks that failed to lift their suspicions of atomic weapons research.

"The Iranian nation has never been seeking an atomic weapon and never will be. It will prove to the world that a nuclear weapon cannot create supremacy," Khamenei told Iranian nuclear scientists, according to a government statement.

"The path of scientific development, particularly the nuclear field, should continue strongly and seriously," he said.

"Pressure, sanctions, threats and assassinations will not bear any fruit and Iran will continue its path of scientific development," Khamenei said.

The forceful restatement of Iran's longheld position came after a five-strong delegation from the UN's International Atomic Energy Agency left empty-handed following two days of talks focusing on suspected military aspects of Iran's nuclear programme.

The delegation's leader, UN chief nuclear inspector Herman Nackaerts, said on arrival in Vienna that, although it had "approached this trip in a constructive spirit," no agreement with the Iranians on elucidating worrisome activities was forthcoming.

"We could not get access" to Iran's military site in Parchin where suspected nuclear warhead design experiments were conducted, according to a November IAEA report, Nackaerts said.

No further talks were arranged, he added, contradicting Iran's envoy to the IAEA, who was quoted by the ISNA news agency as saying discussions "would continue."

"We could not formalise the way forward. We will now report to the (IAEA) director general and later to the board of governors," Nackaerts said. "Then we will have to see what are the next steps."

IAEA Director General Yukiya Amano said in a statement that "it is disappointing that Iran did not accept our request to visit Parchin" during this latest visit, or a previous one at the end of January.

The trip was seen as an important precursor to a possible resumption of talks between Iran and the P5+1 powers --Britain, China, France, Russia and the United States, plus Germany -- which broke down in Turkey 13 months ago.

Western suspicion that Iran is pursuing nuclear weapons research were crystallised -- although not entirely validated -- in the November IAEA report.

Since the report's release, Europe and the United States have been ramping up economic sanctions on Iran, targeting its vital oil exports.

The measures add to four sets of UN sanctions punishing the Islamic republic for not giving timely explanations of its activities.

Speculation has grown that Israel might be poised to launch air strikes on Iranian nuclear facilities, with or without US help.

The Jewish state warns Iran is close to entering a "zone of immunity" in which its nuclear programme will be hidden away in impregnable bunkers.

Iran has reacted defiantly, saying the sanctions will not halt its nuclear activities. It has threatened to strike back at any military action against it, and to maybe close the Strait of Hormuz, the strategic waterway at the entrance to the Gulf, to oil tankers.

This week, it deployed warplanes, missiles and radar facilities in exercises to boost the air defences of its nuclear facilities.

Iran has also announced a halt to the limited amount of oil it exported to Britain and France in retaliation for an EU embargo on its oil due to come fully into effect in July.