US President Barack Obama warned Thursday against imposing further sanctions on Iran, saying a deal would set back Tehran's quest for nuclear weapons capability and prevent the "unintended consequences" of war.

Obama pressed Congress as the UN nuclear watchdog said that Iran has not expanded its nuclear facilities in the past three months in a possible confidence-building measure by the new reform-minded government

"What we have done is seen the possibility of an agreement in which Iran would halt advances on its program," Obama told a news conference a week after talks involving the United States and Iran in Geneva.

"We can buy some additional months in terms of their breakout capacity. Let's test how willing they are to actually resolve this diplomaticly and peacefully," he said.

Obama said that US-led sanctions have "crippled" Iran's economy and that a deal would offer only "very modest relief at the margin." He said an agreement would not, for now, ease the two main measures enacted by Congress -- sweeping restrictions on Iran's vital oil exports and banking system.

Obama said he was telling Congress that his intention "always was to bring the Iranians to the table so we could resolve this issue peacefully."

"No matter how good our military is, military options are always messy, are always difficult, always have unintended consequences, and in this situation are never complete in terms of making us certain that they don't then go out and pursue even more vigorously nuclear weapons in the future," he said.

"If we're serious about pursuing diplomacy, there's no need for us to add new sanctions on top of the sanctions that are already very effective and that brought them to the table in the first place," Obama said.

A halt to nuclear work

In Vienna, the International Atomic Energy Agency said that Iran in the last three months has installed only four new centrifuges at its Natanz plant, compared with 1,861 machines put in place in the previous period.

At the Fordo facility too, which also enriches uranium -- which can be used for a nuclear weapon if highly purified -- no new centrifuges were put into operation, the report seen by AFP showed.

It added that Iran has also not begun operating any new-generation IR-2M centrifuges and that "no... major components" had been installed at a reactor being built at Arak.

The quarterly report, the first since self-styled moderate President Hassan Rouhani took office in Iran in August, comes ahead of a fresh round of talks next week in Geneva between Iran, the United States and five other powers -- Britain, China, France, Germany and Russia.

Secretary of State John Kerry flew in for the last round, which ended with signs of progress but no agreement after three grueling days that ran into the early hours Sunday.

Obama in September spoke briefly by telephone with Rouhani in the first interaction between the two nations' leaders since the 1979 Islamic revolution ousted the pro-Western shah.

Israeli Prime Minister Benjamin Netanyahu, who has had an uneasy relationship with Obama, has vigorously challenged US diplomacy and warned that it was making a "bad and dangerous" deal. Iran and Israel are sworn enemies and have not had relations since the Islamic revolution.

The US Congress, where support for Israel runs deep, is discussing even tighter sanctions on Iran's oil program by removing sanctions exemptions for nations that still buy the country's oil.

Senator Mark Kirk, a leader of the sanctions push, said after talks with Secretary of State John Kerry that a moderate in Tehran is "an Iranian who is out of bullets and out of money."

Obama said it was important to test Iran's willingness for a deal.

Halting further sanctions "gives us ... the opportunity to test how serious are they. But it also gives us an assurance if it turns out six months from now they're not serious, we can dial those sanctions right back up," Obama said.

Iranian hardliners have also attacked Rouhani's outreach, with some arguing that the United States will always be hostile and seek the regime's overthrow.