The agreement - which US officials led by Robert Einhorn and Sandra Kristoff had been negotiating for months - was obtained in a breakthrough on Wednesday in Beijing, administration officials said.

'Nothing is signed, sealed and delivered yet,' said one official, noting the agreement was under review.

Asked if a summit announcement was assured, another official said: 'This is going to depend on what is said between the two heads of state.' The Washington Post yesterday quoted a US official who said that China had given 'very firm, clear and explicit assurances' that it would no longer assist Iran in developing its civilian nuclear power programme.

However, the Post said China offered positive but weaker assurances to US officials that it would stop the sale of cruise missiles to Iran, and limit their transfer of technology and equipment needed to make ballistic missiles and chemical weapons.

At the same time, Mr Clinton is to set up a system to safeguard American nuclear technology sales from being mishandled by China under the planned co-operation agreement.

Although he has decided that Beijing has done enough for him to agree during the Sino-US summit to start allowing the supply of nuclear energy equipment to Chinese firms, Mr Clinton will insist that each transfer be subject to licences from the State or Commerce departments, officials said.

The system for protecting nuclear sales was decided upon after Mr Einhorn and Ms Kristoff returned from Beijing reporting that they judged the Chinese to have made the necessary commitments on stopping their proliferation activities with regard to Iran.

Under the programme, the licences for each individual sale could be revoked or denied if there was evidence that Beijing was violating its pledges on nuclear proliferation.

Similar safeguards are used towards China in the area of sensitive dual-use technology such as supercomputers and advanced electronics.