The European Union has threatened Iran with economic repercussions unless it restores international trust in its nuclear programme.
Tehran denies it has a nuclear weapons programme
EU foreign ministers said a lucrative trade accord could be in danger if Iran fails to meet international concerns over nuclear non-proliferation, fighting terrorism, human rights and the Middle East peace process.
"More intense economic relations can be achieved only if progress is reached in the four areas of concern," the EU ministers said in a statement issued at talks in Brussels.
The International Atomic Energy Agency (IAEA) has given Tehran until the end of October to dispel lingering doubts about its nuclear ambitions.
Tehran insists all its nuclear activity is geared to producing enough electricity from atomic power to meet growing demand and denies it is developing nuclear weapons.
It says weapons-grade uranium found to date came into the country on equipment from abroad.
"The Iranian nuclear programme remains a matter of grave concern for the EU," the ministers said in their statement.
The EU hopes "Iran will sign and implement the additional protocol without delay as a first and essential step to restore international trust in the peaceful nature of its nuclear program," said the ministerial statement.
The 15-member bloc has been conducting talks on a trade accord with the Islamic republic over the past year as the EU pursues a policy of constructive engagement with Tehran.
Many of Europe's leading oil companies have operations in Iran.
Early reports of the threat sparked an angry reaction from the Iranian government on 18 September.
"Iran and the European Union began their political and commercial negotiations based on mutual respect, and just as Iran did not accept preconditions to start the talks, it will very certainly not accept preconditions to continue them," foreign ministry spokesman Hamid Reza Asefi told AFP news agency.
The EU said ministers would review future steps in the light of the next report by IAEA chief Mohamed ElBaradei.
An IAEA visit to Iran was postponed last week after Iranian officials requested more time to prepare for the visit.