Iran and Turkey held secret talks on peace proposals for Syria in 2013 and as recently as this year, but the talks broke down amid mutual suspicions, according to a new report to be published on Tuesday.

The report on the Iran-Turkey relationship by the International Crisis Group (ICG), is based on interviews with top officials. It is being published as pro-regime forces, including Iranian-led militias, storm the last rebel-held districts of Aleppo amid reports of massacres.

The plan envisaged a ceasefire followed by a national unity government and constitutional reform aimed at constraining presidential powers. Most importantly, there would then be presidential and legislative elections under UN supervision. The plan was the subject of several months of shuttle diplomacy between the Iranian foreign minister, Mohammad Javad Zarif, and his Turkish counterpart, Ahmet Davutoğlu, but it eventually collapsed over the future role of the Syrian president, Bashar al-Assad.

“We agreed on every detail, except a clause in the final phase of the plan which called for UN-monitored elections. Turkish leaders wanted Assad barred,” Zarif is quoted as saying in the report. “I noted that this should not be a concern in an internationally monitored election, particularly if, as Turkey holds, Assad has a dreadful record and a minority constituency. But Davutoğlu refused… and our efforts came to naught.”

According to the report, titled Turkey and Iran: Bitter Friends, Bosom Rivals, the Turkish government did not believe that Assad would accept any transition process that would weaken his grip on power and Ankara still thought his military defeat was inevitable.

The Turkish president at the time, Abdullah Gül, told the ICG “our government did not pursue an agreement with Iran because it thought Assad would be toppled in a few months”.

“From Ankara’s perspective, Assad’s battlefield losses would remove need to compromise or at least improve a deal’s terms,” the report said.

The ICG said a second opening for a Tehran-Ankara deal presented itself after the abortive military coup in Turkey in July this year, when Iran promptly stated its support for president Recep Tayyip Erdoğan, creating a temporary thaw in relations and a resumption of talks on Syria. At the same time, military advances by Kurdish YPG forces in northern Syria had led to a simultaneous reconciliation between Ankara and Moscow.

Although the Iranians and Turks still disagreed on Assad’s fate, they focused their discussion on other issues, including whether there should be a presidential or parliamentary system and how power should be shared in general. After two high-level rounds, the report says, the talks got mired in mutual distrust heightened by Turkey’s decision to intervene directly in Syria, in an operation codenamed Euphrates Shield, to prevent the YPG securing all the border zone for the Kurds.

“Iranian officials expressed surprise Turkey had not notified them of the operation despite the presence of a senior Iranian official in Ankara the day before. Turkey may have feared that Iran would tip off the YPG,” the report suggests.