Energy agreements are expected to be a key element of Ahmadinejad's talks with Turkish leaders during his two-day visit to Istanbul beginning on Thursday.
Senior Iranian and Turkish officials told Reuters last week that details, including gas pricing, had been worked out on a project for Turkey to produce and export Iranian gas from the South Pars gas field.
The agreement on those details were to have laid the groundwork for the signing of parts of the deal with Ahmadinajad, but new demands from Tehran have slowed down the process.
"A text regarding the agreement over gas with Iran had been set out, but demands made by the Iranians over the past few days are unacceptable," said a source from Prime Minister Tayyip Erdogan's office.
"If Ahmadinejad does not accept the accord on which agreement was reached, the accord will not be signed during this visit," he said.
Last July Turkey and Iran signed a preliminary agreement to export Iranian gas to Europe through Turkey, including a provision for Turkey to produce 20.4 billion cubic metres of natural gas in Iran's huge South Pars gas field.
The investment for Turkey's gas production in Iran is seen at approximately $3.5 billion.
The United States has voiced its opposition to the deal amid a standoff between Iran and the West over Tehran's disputed nuclear enrichment programme.
Turkey, which is a net importer of gas and oil, has been trying to find a way to use its geographical position between Europe and the energy-rich Caspian to import gas and sell it on to Europe.