The point made by Yukiya Amano, director-general of the International Atomic Energy Agency, reinforced the view of many analysts that increased Western economic pressure on Iran has failed to make it change its nuclear course.
He spoke a day before senior officials from six world powers were to meet in Brussels to weigh strategy towards Iran amid signs of a renewed push to resolve the dispute diplomatically after US President Barack Obama's re-election.
World powers, which first ushered in UN sanctions on Iran back in 2006, are concerned Israel may try to bomb Iranian nuclear sites without a peaceful resolution to the row soon.
The United States and its Western allies have sharply ratcheted up punitive steps on Iran this year to target its vital oil exports, hoping this will convince the country to finally back down in a stand-off that has raised fears of war.
But asked whether sanctions had produced any deterrent effect, Amano told reporters in Paris: “We are verifying the activities at the nuclear sites in Iran and we do not see any effect. They are, for example, producing enriched uranium up to 5 percent and 20 percent with a quite constant pace.”
Amano, whose inspectors regularly visit Iran's nuclear facilities, added: “It has not changed. We have observed that the progression of enrichment has been constant. There has been a steady, gradual increase in the amount.”
The IAEA, tasked with preventing the spread of nuclear weapons in the world, said in its latest quarterly report on Iran that it had finished installing enrichment centrifuges at its Fordow underground plant.
The report, submitted to IAEA member states on Friday, underlined the tough task facing the powers in seeking to persuade Tehran to suspend work which Iran says is peaceful but they fear is aimed at developing a nuclear weapons capability.
It is a “very troubling report,” a senior Western official in Vienna, where the IAEA is based, said.
Iran's envoy to the IAEA, Ali Asghar Soltanieh, told Reuters that the report proved Tehran's nuclear program is “exclusively for peaceful purposes.”
Refined uranium can be used to fuel nuclear power plants, Iran's stated aim, but can also provide the explosive core of a nuclear warhead, which the West fears is its ultimate aim.
The six powers -- the United States, France, Germany, Britain, Russia and China -- have failed to achieve a breakthrough in three rounds of talks with Iran since April.
But neither side has been willing to break them off, in part because of concerns that this could lead to a new, devastating conflict in the Middle East if Israel attacked its arch-enemy, which rejects the Jewish state's existence.
The Western official said this week's meeting in Brussels would discuss whether to update the powers' previous offer to Iran and seek to “lay the groundwork” for a new round of talks which could happen before year-end.
“We want to try to give Iran incentives to meet its obligations. But Iran will obviously have to take steps as well,” the official said. “It is looking to see what we can do to help bring Iran back to the table to negotiate seriously.”
At the meetings earlier this year, the powers demanded that Iran halt its higher-grade enrichment, close down Fordow and ship out its stockpile of the material. In return, they offered limited incentives focused on technology cooperation.
Iran has signaled it may be ready to discuss its enrichment to a fissile concentration of 20 percent -- which advances Tehran much of the way towards bomb-grade material, but which it says is meant to provide fuel for a medical research reactor -- but wants an immediate lifting of sanctions in return.
That demand has been rejected by the West.
In Tehran, Iranian Foreign Ministry spokesman Ramin Mehmanparast said the powers should recognize Iran's “right to peaceful nuclear activities” -- code for uranium enrichment.
“We are always ready for constructive and principled negotiations,” Mehmanparast said.