Turkey is not a suitable location for nuclear talks expected to be held by major powers in a few weeks, according to Iran.
As the neighbours clashed over unrest in Syria, Iran's ally, Alaeddin Borujerdi, head of the Iranian parliament's national security and foreign policy committee, said: "Given Turkey's unprincipled policies, it isn't beneficial for the upcoming negotiations between Iran and the 5+1 countries to be held in Istanbul."
Talking to the Tehran-based Etemaad newspaper, he added: "Iranian lawmakers have many times said that negotiations need to take place in another spot and in a country that is a friend of Iran."
Mr Borujerdi's remarks came after Hillary Clinton, the US secretary of state, revealed last week that negotiations between Iran and the five permanent members of the United Nations Security Council plus Germany would be held from April 13 to 14 in Istanbul.
Turkey is playing a leading international role in trying for force the removal of the Syrian president, Bashar Al Assad, from power.
Iran this week suggested alternative venues in countries it deemed friendlier, among them Iraq, Lebanon, China and even conflict-torn Syria. It did so knowing that all are unacceptable to western powers for either political or security reasons.
Scathing anti-Turkish remarks by senior Iranian officials have infuriated Ankara, which has supported Tehran's right to a peaceful nuclear programme and opposed unilateral US and European sanctions against Iran.
Turkey's foreign ministry summoned Iran's ambassador in Ankara on Wednesday to express "dismay" over strident rhetoric by leading Iranian parliamentarians who condemned Turkey for hosting a "Friends of Syria" conference last weekend.
That meeting brought together myriad Syrian opposition groups and western countries demanding Mr Al Assad's removal.
Iran's powerful parliamentary speaker, Ali Larijani, criticised the "Friends of Syria" gathering as supporters of Israel.
Mr Borujerdi went further, saying on Wednesday: "Taking into account the extremist and illogical position of Turkey on Syria and the recent conference on Syria, Turkey has de facto lost any competence to hold host the [nuclear] meeting."
Other Iranian officials have accused Ankara of being "Nato's pawn" in the Middle East with "neo-Ottamist" ambitions".
Turkey's prime minister, Recep Tayyip Erdogan, visited Iran last week to press the Islamic republic to end its support for Mr Al Assad.
He made little progress in a meeting with Iran's absolute supreme leader, Ayatollah Ali Khamenei, who told him the Islamic republic would continue to "defend Damascus" so it can continue its "resistance" against the "Zionist regime" [Israel]."
The following day Turkey said it would reduce the amount of oil it buys from Iran by 10 per cent, ceding to US pressure.
Even so, the Iranian regime was flattered by Mr Erdogan's visit, which it saw as an acknowledgement of Tehran's role as a regional power broker despite determined American attempts to isolate the Islamic republic.
That flattering perception will be enhanced by yesterday's announcement that Kofi Annan, the international peace mediator on Syria, will visit Tehran on Wednesday for talks on Syria.
Iran views the former UN secretary general as a respected and neutral arbiter. Tehran has endorsed Mr Annan's peace plan on Syria, which calls for a national dialogue and a ceasefire but not, crucially from Iran's point of view, for Mr Al Assad to step down.
In turn, Tehran will be required to demonstrate that it can exert genuine pressure on Mr Al Assad to implement long-promised political reforms.
Iran, though, is sending mixed signals to the international community, reflecting divisions within its own regime.
Iran's foreign minister, Ali Akbar Salehi, has made it clear Turkey that is still his favoured choice for nuclear talks with the six world powers.
Other powerful Iranian figures are opposed.
"Given that our Turkish friends reneged on some agreements, it is better that Iran's talks ... are held in a friendly country," said Mohsen Rezaie, a former commander of Iran's elite Revolutionary Guard. He is now a spokesman for Iran's Expediency Council, a body that advises Ayatollah Khamenei.
Western officials suspect Iran's 11th-hour objections to Istanbul as a venue are brinkmanship, with Tehran determined to show it is entering the nuclear negotiations from a position of strength. If so, Iran would present its eventual acceptance of Istanbul as a "concession", demanding in turn a reciprocal measure from the West.
Others wonder if Iran is pulling back altogether from the nuclear negotiations.
There are similarly conflicting voices from the Iranian regime about Saudi Arabia, the Islamic republic's main regional rival which is a staunch supporter of the Syrian opposition and a vocal opponent of Tehran's nuclear ambitions.
Ali Akbar Hashemi Rafsanjani, a former Iranian president and pillar of Iran's 1979 Islamic revolution, said this week that Tehran should forge better relations with Riyadh to counter western sanctions on Iranian oil exports.
Saudi Arabia, a US ally and Opec's biggest producer, is facilitating western sanctions by pledging to tap its spare capacity to make up for any imposed shortfall of Iranian oil exports.
Iranian hardliners promptly hit back against Mr Rafsanjani's recommendation to cosy up to Riyadh, declaring he "has no wisdom".
* With additonal reporting by Bloomberg