"We will never yield to pressure ... we will not accept unreasonable demands ... Iran will not give access to its (nuclear) scientists," Khamenei said according to the state TV.

Earlier, Iran had made it clear that it would only accept a deal over its contested nuclear programme if world powers simultaneously lifted all sanctions imposed on it.

A tentative deal between Iran and the six world powers was reached in Switzerland on April 2, and aimed at clearing the way for a final settlement on June 30. Discussions will resume on April 21.

However, different interpretations have emerged over what was agreed in the framework, suggesting that nailing down a final agreement will be tough, even without the added complication of Congress.

Many Congressmen have been highly critical of the U.S.-led negotiations, supporting Israel, which has said the framework proposal will not prevent Iran from developing atomic arms. Tehran says its nuclear programme is for peaceful purposes.

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