Turkey's prime minister, Recep Tayyip Erdogan, travels to Tehran today where he will press a sceptical Iran to stop supporting the Al Assad government in Syria.
Iran's nuclear programme will also be high on his agenda.
Mr Erdogan will try to persuade Tehran that the United States is committed to a diplomatic solution to the nuclear crisis and that Israeli threats of military action are not a bluff, analysts and diplomats said.
It is a tricky balancing act for the Turkish premier. While cajoling Iran on the nuclear and Syrian issues, he will assure his hosts that Turkey is keen to expand mutually beneficial trade relations and keep diplomatic ties cordial.
Turkey receives 7 per cent of Iran's oil exports and in turn exports many manufactured goods to Iran. Both countries, diplomats say, are eager to insulate their commercial interests from political differences.
"Erdogan will try to persuade Iran to persuade Assad to leave," said Hugh Pope, the Turkey project director of the International Crisis Group. "He will push the idea that there has to be a transition in Syria, while the Iranians will presumably try to seek assurances that whatever happens in Syria, their interests will be protected."
If Al Assad's departure proves too ambitious a goal, Mr Erdogan will at least seek assurances that Iran "won't disagree with agreed international action on Syria," said Scott Lucas, an Iran expert at Birmingham University in England. "You'd hope that Iran is beginning to distance itself from Syria and encourage Assad not to just talk about reforms but to do something."
Turkey will host a "Friends of the Syrian People" conference on Sunday, attended by some 60 countries. The meeting will focus on supplying the Syrian opposition with medical supplies and communications support, described as "non-lethal aid".
Iran, meanwhile, is providing an array of assistance to Mr Al Assad's regime to help him suppress the year-old anti-government protests, from high-tech surveillance technology to guns and ammunition, American and European officials say.
Iran has a huge interest in the survival of Mr Al Assad's regime, Tehran's only ally in the Arab world. His removal could sever Iran's umbilical cord to Hizbollah, Tehran's Shiite coreligionist ally in Lebanon, which gives the Islamic republic a presence on Israel's northern border and enables Iran to project its power in the region.
Tehran is already smarting from the recent decision by the militant Palestinian group, Hamas, to distance itself from Syria and Iran and move closer to Turkey, Egypt and Qatar.
The Turkish premier's Iran trip follows talks with Barack Obama, the US president, on the sidelines of a nuclear security summit in South Korea this week. "If there are messages from Washington, Erdogan will transmit them," Mr Pope said.
Mr Erdogan told Mr Obama he would try to persuade Iran to halt its backing of the Al Assad regime, Ben Rhodes, the US Deputy National Security adviser, said on Sunday.
"Erdogan's also likely to offer his services on the nuclear file," a Western diplomat in Tehran said. Turkey, which has ambitions to be a broker in international affairs, is keen to host long-stalled nuclear talks between Iran and six world powers that diplomats say will resume on April 13, although the venue has yet to be agreed.
Two years ago, Mr Erdogan would have been assured of a warm reception in Tehran, which trusted Turkey enough to consider it as a home for Iran's stockpile of enriched uranium under a deal - which later collapsed — to resolve the nuclear stand-off.
But ties have been strained by Ankara's robust support for the Syrian opposition and competition for influence in a changing Arab world where Turkey's star is rising while Iran's is declining.
Iran was infuriated by Ankara's agreement in September to deploy a Nato missile early-radar warning system in Turkey which Tehran maintains is a US ploy to protect Israel from any counter-attack if it target Iran's nuclear facilities.