World powers have agreed a draft resolution, which expresses "increasing concern" over Iran's controversial nuclear programme.
The document by the five permanent members of the UN Security Council and Germany comes amid fears of "possible military dimensions" to the programme.
The text is to be discussed at a meeting of the UN nuclear watchdog, the IAEA, later this week.
The IAEA proposes sending a high-level mission to Iran to address the fears.
Tehran says its programme is for peaceful purposes.
The draft resolution was hammered out by Britain, China, France, Russia, the US and also Germany on Thursday in Vienna, as the International Atomic Energy Agency continues to debate the latest report on Iran released last week in the Austrian capital.
The draft expresses "deep and increasing concern about the unresolved issues regarding the Iranian nuclear programme, including those which need to be clarified to exclude the existence of possible military dimensions".
The document also "calls on Iran to engage seriously and without preconditions in talks aimed at restoring international confidence in the exclusively peaceful nature" of Tehran's programme.
However, it stops short of reporting Iran to the UN Security Council, which could impose more sanctions, the BBC's Bethany Bell in Vienna reports.
IAEA Director General Yukiya Amano earlier said that there might be undeclared nuclear material and activities.
"Our technical experts have spent years painstakingly and objectively analysing a huge quantity of information from a wide variety of independent sources, including from a number of member states, from the agency's own efforts and from information provided by Iran itself. The agency finds the information to be, overall, credible," Mr Amano said in a statement.
"It is consistent in terms of technical content, individuals and organisations involved, and timeframes. The information indicates that Iran has carried out activities relevant to the development of a nuclear explosive device."
Mr Amano expressed hope that a date for the the proposed mission to Iran would be agreed soon.
The US and its allies want to see stronger sanctions imposed on Iran, but Russia believes the report contains no new evidence - and could hurt the chances for diplomacy, our correspondent says.
China says sanctions cannot resolve the issue.
On Wednesday, Iranian General Hassan Firouzabadi said that - contrary to speculation - the US and Israel were not behind a weekend munitions base blast that killed 17 Revolutionary Guards, including a key ballistics missile expert.
"This recent incident and blast has no link to Israel or America, but the outcome of the research, in which the incident happened as a consequence, could be a strong smack to the mouth of Israel and its occupying regime," Gen Firouzabadi was quoted as saying by the student news agency Isna.
Iranian officials had previously said the accident happened while munitions were being moved at the base, without linking it directly to weapons research.
Brig Gen Hassan Moqaddam, who was considered a key figure in Iran's missile programme, was the most senior casualty in the incident.