Esmaeil Baghaei said on Saturday that the signing of the Islamabad memorandum would not take place on Sunday, according to Iranian state media.
Baghaei said the possibility of signing the memorandum in the coming days could not be ruled out, but cautioned against speculation regarding the timing of the agreement.
He said uncertainty remained due to what he described as hesitation on the part of the other side.
The remarks came a day after Prime Minister Shehbaz Sharif said the United States and Iran had agreed on a framework for a peace deal after more than three months of conflict and were expected to sign an initial agreement within 24 hours.