Tehran says its nuclear programme is for peaceful purposes
European Union foreign policy chief Javier Solana has given Iranian leaders a stern warning to accept snap inspections of its nuclear programme.
He said at a news conference with the Iranian foreign minister in Tehran that it would be "bad news" for Iran - and for the EU - if it did not sign an additional protocol to the Nuclear Non-Proliferation Treaty.
A leaked report earlier this week revealed the International Atomic Energy Agency's unease at finding traces of highly-enriched uranium at the Natanz nuclear facility.
Enriching uranium is a way of purifying it so it can be used in nuclear fuel or in weapons.
Last month, the EU warned it could review its ties with Iran in September in light of the issue.
Mr Solana said that only if there were rapid progress in Iran's discussions with the IAEA would it be possible to avoid what he called unwelcome effects on the EU's relations with Iran.
The BBC's Jim Muir in Tehran says that means not only signing the additional protocol, but also coming up with answers to a number of questions raised by the IAEA, especially relating to evidence that Iran may have been processing weapons-grade uranium.
Allows for inspections at short notice
IAEA can take environmental samples at any location
If progress is not made soon on those and other issues, some EU member countries have made it clear that negotiations towards a trade and co-operation agreement with Iran will go on hold.
Earlier this week, Iran said it was ready to start negotiations but wanted guarantees that inspectors would not be given total freedom of movement or violate military secrets.
Tehran insists its programme is to generate electricity and is for peaceful purposes only, to satisfy its growing demand for power and prevent long-term energy shortages.