Iran on Tuesday set September 1 as a possible date to resume nuclear talks with six world powers that have been stalled since October, but insisted its conditions must first be met.

Washington responded saying it was willing to meet Iran over its nuclear programme if Tehran's offer was "serious."

According to the official IRNA news agency Iran's top nuclear negotiator Saeed Jalili sent a letter to European Union foreign policy chief Catherine Ashton saying Tehran needed three issues clarified by the world powers before it could consider resuming talks.

Ashton, who is negotiating with Iran on behalf of the so-called P5+1 powers -- Britain, China, France, Russia, the United States and Germany -- invited Tehran for talks after the UN Security Council imposed new sanctions on the Islamic republic on June 9.

Jalili said the world powers must answer whether the talks are aimed at "engagement and cooperation or continued confrontation and hostility towards Iranians."

"Will you be committed to the logic of talks which calls for avoiding threats and pressure?" he asked, calling on the six powers to air a "clear view" on the "Zionist regime's nuclear arsenal."

"Your response to the above questions can pave the way for forming talks to allay common global concerns for peace and justice with the presence of other interested countries from September 1," Jalili told Ashton.

Israel, which has the Middle East's sole if undeclared nuclear arsenal, has backed US-led efforts to prevent Iran developing a nuclear weapons capability through sanctions, but has also refused to rule out military force.

In Washington, State Department spokesman Mark Toner told reporters: "If Iran is serious about talking to the P5-plus-one, then I think we're willing to meet."

"Obviously we'd have to evaluate the Iranian offer," Toner added.

Also on Tuesday President Barack Obama vowed the United States would continue to put pressure on Iran as he received Israeli Prime Minister Benjamin Netanyahu at the White House.

"We intend to put pressure on Iran to meet its international obligations and to cease the kinds of provocative behaviour that have made it a threat to its neighbors and the international community," Obama said.

Netanyahu for his part called for "much tougher" sanctions against Tehran.

Iran insists that its nuclear programme is aimed solely at peaceful purposes and says that the international community should focus on Israel, which, unlike Iran has not signed the nuclear Non-Proliferation Treaty.

On June 28, President Mahmoud Ahmadinejad declared a two-month freeze on talks with world powers over Iran's nuclear programme, saying it was a "penalty" for backing new UN sanctions on Tehran.

Iran and the world powers had previously held talks in Geneva on October 1 on Tehran's nuclear programme.

Ashton's spokeswoman Maja Kocijancic welcomed Iran's readiness to talk but told reporters in Brussels that the dialogue "would have to be on the nuclear programme of Iran."

Meanwhile Kazem Jalali, a lawmaker and member of parliament's foreign policy committee, said on Tuesday that the presence of Brazil and Turkey in the talks could be "beneficial."

But he said that a "roadmap is more important to the talks than new countries," Mehr news agency reported.

Brazil and Turkey have sealed a deal with Iran aimed at facilitating a nuclear fuel swap with Russia and France. But talks with world powers on a swap have also been deadlocked since October.

Also on Tuesday Iran rejected rejected claims by some of its officials that airports in Britain, Germany and the United Arab Emirates had refused to refuel its passenger planes, in line with latest US sanctions on Tehran.

burs/hkb