World powers urged Iran to cooperate with the UN atomic watchdog Wednesday, otherwise it could soon face further censure for blocking the investigation into its controversial nuclear drive.

"We call on Iran to cooperate fully with the (International Atomic Energy) Agency," the so-called P5+1 grouping said in their first joint statement to the IAEA's board of governors in two years.

The P5+1 comprises the five permanent members of the UN Security Council -- Britain, China, France, Russia, the United States -- plus Germany.

It is talking with the Islamic republic to allay fears over its nuclear programme, which Tehran insists is entirely peaceful, but which the West believes masks a covert weapons programme.

The last time the six powers issued a joint statement to the IAEA's 35-member board of governors was in March 2009.

Since then, the group has held two rounds of talks with Tehran, in Geneva in December and in Istanbul in January, but which produced no concrete breakthrough in the long-running standoff.

"We came to Geneva and Istanbul with a constructive spirit and proposed in Istanbul several practical ideas aimed at building confidence and to facilitate the engagement of a constructive dialogue with Iran on the basis of reciprocity and step-by-step approach," said Russian governor Grigory Berdennikov on behalf of the P5+1 at an ongoing IAEA meeting at its Vienna headquarters.

While no "substantive result" was reached in Istanbul, "we look to Iran to engage in future in a similarly constructive spirit," Berdennikov said.

"We remain ready to participate actively in the process with Iran. We expect Iran to demonstrate a pragmatic attitude and to respond positively to our proposal and to our openness toward dialogue and negotiations. The door remains open," he said.

On Monday, IAEA director general Yukiya Amano had complained that Iran was "not providing the necessary cooperation" and, in particular, was stonewalling questions about alleged weaponsiation studies.

In a statement on behalf of the 27-nation EU, Hungary expressed "serious concern" about Iran's refusal the so-called "alleged studies" since August 2008.

Hungary suggested that "it might be helpful if (Amano) could provide the board with a comprehensive analysis on possible military dimensions to Iran's nuclear programme."

In his latest report on Iran, the director general had revealed that the IAEA was in possession of "new information" that the weaponisation work -- including uranium conversion, high explosives testing and the adaptation of a ballistic missile cone to carry a nuclear warhead -- may have gone on beyond 2004, which is more recently than initially thought.

So far, Iran has dismissed the evidence as "fabricated", but refused to discuss the matter any further.

In his own statement to the board Wednesday, Iranian ambassador Ali Asghar Soltanieh took a simliar line.

The US and the EU "have once again repeated the unfounded assertion questioning the exclusive peaceful nature of our nuclear activities without presenting any authenticated evidence," Soltanieh complained.

In response to the P5+1 statement, the Iranian ambassador said Iran "is fully prepared to continue negotiations for coooperation on common elements with the P5+1 (and) is ready to start negotiations as soon as P5+1 is ready.

"I advise the group to seize this unique opportunity to change the gear from confrontation to cooperation and to come to negotiating table without further delay. The door is still open," Soltanieh said.

His US counterpart, Glyn Davies, warned that Iran could soon find itself censured by the IAEA board of governors if it continued to block the IAEA investigation.

"For this board, the decision was made to let the report and let the words of the members of the board of governors speak for themselves, rather than to run a resolution at this stage," Davies told reporters on the sidelines of the meeting.

"We'll see what happens in June. The last resolution we ran was in late 2009. But I can see that time coming very quickly when we may need to do that again in order to even more formally underscore our concern about the Iranian case," he said.