Iran's main nuclear negotiator Saeed Jalili agreed in phone talks on Thursday to speak again with world powers before the month is out on Tehran's contested programme, the EU's top diplomat said.

"I proposed, and Dr Jalili agreed, that we talk again after further reflection at the end of the month," Catherine Ashton said in a statement after discussing "how to move the talks with Iran on the nuclear issue forward".

She did not say if these next talks would be face-to-face or more phone contact, but underlined: "I impressed the need for Iran now to address the issues we have raised in order to build confidence."

Working on behalf of Britain, China, France, Germany, Russia and the United States, European Union High Representative Ashton is trying to elevate dialogue with Tehran into a stable exchange after tortuous stop-start diplomacy.

The planned contact will follow face-to-face talks between the pair's respective deputies in Istanbul in Turkey last week.

Ashton said she had "explored diplomatic ways to resolve international concerns about Iran's nuclear programme", which Israel and the West suspect is a cover for efforts to build an atomic bomb, a charge denied by Iran.

The so-called P5+1 group she represents has told Iran to immediately stop enriching uranium to the 20 percent level, to ship out its existing 20 percent stock and to shut down a fortified underground enrichment facility.

Experts say uranium must be enriched to 90 percent purity to make nuclear bombs, but that 20 percent purity is a key stage in paving the way towards that capacity.

Iran insists it has a right to uranium enrichment under the nuclear Non-Proliferation Treaty and wants Western sanctions on its economy to be eased.

The US Congress on Wednesday approved punishing new sanctions targeting Iran's energy and shipbuilding sectors, amid deliberation over a possible first strike against Iran's nuclear facilities by Israel.

Israeli Prime Minister Benjamin Netanyahu has complained that "all the sanctions and diplomacy so far have not set back the Iranian programme by one iota".