There has been a cautious response to a deal Iran has struck with Brazil and Turkey to send some low enriched uranium abroad, in return for higher grade nuclear fuel for a research reactor.
President Dmitry Medvedev of Russia and the United States have expressed concern at Iran's statement that it intends to continue to enrich uranium.
The White House says Iran must give an authoritative statement of its plans to the International Atomic Energy Authority.
The German government said that nothing could replace a deal between Iran and the IAEA.
Iran backed out of a similar exchange proposal last October.
The West suspects that Iran's nuclear programme is aimed at making weapons. Tehran denies this and says it is for domestic power.
Under the deal, Iran's foreign ministry said it was ready to ship 1200kg of low-enriched uranium to Turkey, in return for fuel for a research reactor.
In return, Iran says it expects to receive 120kg of more highly enriched uranium (20%)
Iran's stocks are thought to be much larger than the 1200kg covered by the agreement.
The BBC reports final stages of a fourth sanctions package are being negotiated by UN Security Council members.