Iran's announcement that it might build two new uranium enrichment plants inside mountains to avert air strikes has drawn a sharp reaction from Washington.

But signals from the European Union were less clear.

Monday's announcement from Iran's atomic chief Ali Akbar Salehi came just a day after top US General David Petraeus warned that Washington would pursue a "pressure track" against Iran to thwart its nuclear programme.

Salehi said the enrichment capacities of the new sites would be similar to the existing facility in the central city of Natanz, where a defiant Tehran is refining uranium despite three sets of UN sanctions.

The new plants will be equipped with new generation centrifuges, which rotate at supersonic speed to enrich uranium, and the facilities would be hidden in mountains to protect them from "any attacks," he added.

He was referring to two of the 10 new plants President Mahmoud Ahmadinejad announced last November that Iran would build.

And that move came despite international condemnation of Iran's work on a second enrichment plant inside a mountain near the Shiite holy city of Qom, intended as a back-up should the one in Natanz be targeted in air strikes.

Tehran plans to have it operational by 2011, the Islamic Republic told the IAEA in a letter dated October 28, 2009.

Last Thursday, in a blunt first report to the watchdog's board of governors, IAEA chief Yukiya Amano expressed concern that Iran might be seeking to develop a nuclear warhead, confirming the fears of many western powers.

"The IAEA report represents one of the clearest denunciations of what the Iranians have been working on," White House spokesman Robert Gibbs told reporters.

US President Barack Obama and international leaders "have been clear that without a change in behavior, the Iranian government faces necessary consequences," he added.

He even suggested that China -- which has balked at punitive measures -- would eventually endorse a fourth round of UN sanctions against Iran.

On Sunday, Petraeus said the United States, which along with its ally Israel has not ruled out military strikes against Iran's nuclear sites, would increase pressure on Tehran.

But Admiral Mike Mullen, chairman of the Joint Chiefs of Staff, told reporters on Monday that "no strike, however effective, will be in and of itself decisive."

Mullen and other top US officials have previously refused to rule out military action against Iran.

"But, as I have said many times, I worry a lot about the unintended consequences of any sort of military action.

"For now, the diplomatic and the economic levers of international power are and ought to be, the levers first pulled," Mullen said.

The US military nevertheless stood ready with plans for possible action if necessary.

Mullen returned last week from a Middle East tour that took in Egypt, Israel, Jordan, Saudi Arabia and the United Arab Emirates. Israeli Defense Minister Ehud Barak visits Washington this week.

In Brussels, there were mixed signals from the member states of the European Union.

"Unhappily, all the actions by the Iranian side for weeks confirm that we must move to (more) sanctions," French European Affairs Minister Pierre Lellouche said.

But some of his EU counterparts said diplomacy had not run its course and insisted on the need for a UN Security Council decision.

"There is no solution other than the diplomatic route, we must try all possible diplomatic actions," said Luxembourg's Foreign Minister Jean Asselborn.

His Swedish colleague, Carl Bildt, insisted that the way forward lay with international action through the United Nations.

"I don't want to undercut the discussions going on in the UN at the moment," he told reporters as he arrived for the Brussels talks.

"It's only by having an agreement with everyone, and everyone includes China, Brazil, India, Turkey, that you can have an impact."

Israeli Prime Minister Benjamin Netanyahu meanwhile insisted that "now" was the time for new sanctions on Iran's oil exports.

But Turkish Prime Minister Recep Tayyip Erdogan warned in a newspaper interview that any pre-emptive Israeli strike on Iran's nuclear facilities would be a "disaster of unpredictable consequences."

"We have to find a solution through diplomatic means," he told the Spanish daily El Pais. "We must not leave the negotiating table."