For example, the tone of Iran's most recent letter has reportedly been more conciliatory than earlier ones, and the letter didn't impose any conditions for the resumption of talks.

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The resumption of normal operations has been further delayed, and April deliveries will be affected, Imapala said in a letter to clients.

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Last week, Iran's nuclear negotiator, Saeed Jalili, sent a letter to the EU's foreign-policy chief, Catherine Ashton, calling for an immediate resumption of talks without preconditions.

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