Britain, France, Germany and the United States raised the new developments at the UN Security Council. They are however unlikely to get Russia and China to agree to a fifth round of nuclear sanctions against Iran.

Enrichment at the Qom facility was "a further clear breach" of the six Security Council resolutions passed on Iran, said Britain's deputy ambassador Philip Parham.

Western nations raised concerns at the council because the new plant has no "credible civilian use" and was "a further violation" of UN Security Council and International Atomic Energy Agency resolutions, added France's deputy envoy Martin Briens.

The UN Security Council has agreed four rounds of sanctions against Iran over its nuclear drive, which the Western allies say hides a covert effort to build a nuclear bomb. Iran denies the charge.

Germany's deputy UN envoy Miguel Berger called the Iranian enrichment of higher grade uranium at Qom "a step of escalation". Joining her European counterparts, US deputy ambassador Rosemary DiCarlo said Iran has "no justification for enriching uranium at this level."

European and US diplomats said Russia and China had also expressed concern over Iran's latest move at Wednesday's meeting.

But Russia and China, which as permanent members can veto any council measure, are likely to block any new sanctions move despite the escalating confrontation between Iran and its international foes, diplomats acknowledge.

As the European Union and United States have stepped up their individual sanctions against Iran, the Tehran government has threatened to close the strategic Strait of Hormuz oil shipping lane.

"We will work with our partners to consider what the next steps are going to be," said Briens when asked about what the Security Council would do.

The Western nations reaffirmed their policy of seeking talks with Iran while pressuring the Islamic republic with sanctions.

"What is important is that the dual track is open at the moment," Parham told reporters as he urged Iran to return to talks.

"We keep on trying to get serious negotiations to start but so far Iran has not responded," said the French envoy.

The Iranian leader arrived in Cuba for talks with his counterpart Raul Castro as the Islamic republic blamed Israel and the United States for the killing of a nuclear scientist in a Tehran car bombing.

The scientist's killing heightened already high tensions with the West over Iran's suspect nuclear program, but a defiant Ahmadinejad flashed the victory sign several times after landing in Havana.

The Iranian leader was greeted by nine girls in traditional Iranian attire as he stepped off the plane that flew him in from Nicaragua on the third leg of his Latin American tour, aimed at shoring up ties in the region.

Ahmadinejad was taken to the University of Havana where he picked up an honorary doctorate and told students in the Americas' only Communist country that capitalism could soon be on its last legs.

"Our shared task, mission and challenge is to make a great effort for justice to be achieved, (otherwise) millions will suffer injustice," Ahmadinejad said.

"We are watching the capitalist system decay... it is heading toward a dead end," he said, stressing that what is needed is "a new order, a fresh look, that respects all human beings, a way of thinking that is based on justice."

Ahmadinejad repeatedly took swipes at the United States but did not address the rising tensions over Tehran's nuclear program, which the West says is aimed at developing weapons -- a charge Iran denies.

"Today the only thing capitalism does is kill. It is a failed system in decay," he said.

Ahmadinejad's arrival came hours after Mostafa Ahmadi Roshan, a 32-year-old nuclear scientist, was killed in Tehran.

Iran pointed the finger at Israel and the United States, which have vowed to prevent Iran from building a nuclear weapon.

Iran's Vice President Mohammad Reza Rahimi told state television the killing, which occurred in front of a university campus in east Tehran, would not stop Iran making "progress" in its nuclear activities.

Iranian officials said the assassination method -- two men on a motorbike attaching a magnetic bomb to the target's vehicle -- was similar to that used in the killings of three other scientists over the past two years.

Washington denied any involvement in the killing.

"The United States had absolutely nothing to do with this. We strongly condemn all acts of violence, including acts of violence like this," said National Security Council spokesman Tommy Vietor.

The killing came after confirmation by the UN nuclear watchdog IAEA that Iran had begun enriching uranium in a new, underground bunker, a development that the United States, Britain, France and Germany denounced as an unacceptable violation of UN Security Council resolutions.

Faced with growing pressure from the West, Ahmadinejad has been seeking support from four Latin American countries hostile to Washington.

Cuba and Iran share similar positions in international organizations, with Tehran condemning the half-century US trade embargo against Cuba and Havana recognizing Iran's right to develop nuclear energy for peaceful purposes.

Israel, Tehran's sworn enemy, has threatened to launch air strikes on Iran's nuclear facilities. The United States has said "all options are on the table" in terms of dealing with Iran -- including military action.

Tehran has meanwhile threatened to close the strategic Strait of Hormuz at the entrance to the Gulf if it is attacked. Twenty percent of the world's tanker-carried oil flows through the strait.