It is Ahmadinejad's first official visit to Turkey as Iranian leader
Iran's leader Mahmoud Ahmadinejad has held talks with Turkish President Abdullah Gul during his first official trip to Turkey since taking office.
The trip is seen as a sign of closer ties between the two countries, whose relations have improved since Turkey's governing AK party took power in 2002.
But there has been no word on an expected deal to supply more Iranian gas to Turkey - a Nato and US ally.
Mr Ahmadinejad will also meet PM Recep Tayyip Erdogan during his two-day trip.
The visit is also his first bilateral visit as president to a Nato member country.
It comes amid continuing international tension over Iran's nuclear programme. Turkey appears keen to act as a go-between between Iran and the West over the issue, correspondents say.
The energy deal might not now be signed during Mr Ahmadinejad's visit following new demands from the Iranian side, a source from the Turkish prime minister's office told Reuters news agency.
The US state department had warned Ankara not to strike a deal with Iran that would undermine diplomatic efforts to halt Tehran's nuclear programme, the UK's Financial Times newspaper reports.
After Russia, Iran is said to be the biggest provider of gas to Turkey.
The visit had already provoked controversy in the press over the usual protocol of foreign leaders visiting the mausoleum of the founder of the Turkish republic, Mustafa Kemal Ataturk, says the BBC's Pam O'Toole.
Turkish media have speculated Mr Ahmadinejad is holding talks in Istanbul rather than Ankara to sidestep paying respects to Ataturk, a champion of secularism.
Turkish officials say the talks are expected to focus on bilateral ties, regional and international issues.
Relations between Turkey and Iran have not always been entirely cordial, our correspondent adds.
Although Turkey is predominantly Muslim, in the past, the staunchly secular Turkish establishment has suspected Iran of trying to export its Islamic revolution to neighbouring countries.
And Ankara is a close ally of Tehran's arch enemies, the US and Israel; Israeli officials have made clear their unhappiness with the Iranian president's visit.
But in recent years relations between Tehran and Ankara have improved dramatically.
They have considerable trade ties and Ankara's begun purchasing Iranian gas via a pipeline between the two countries, despite Washington's disapproval.
The two countries are also likely to discuss their growing co-operation in the fight against Turkey's Kurdish militant group, the PKK, and its sister organisation, PJAK, which launches attacks against Iran.
And Turkey, which supports Iran's right to nuclear power for peaceful purposes, is likely to renew its offer to facilitate negotiations between Iran and six world powers over Tehran's controversial nuclear programme.