Ankara (Agencies): Russian President Vladimir Putin has met Turkish PM Recep Tayyip Erdogan for talks expected to include the Syrian conflict, over which they disagree.
Moscow is a key ally of Syria, while Ankara is backing the rebels trying to oust Syrian President Bashar al-Assad.
In October, Turkey intercepted a Syria-bound plane which it said had Russian-made defence equipment - a claim rejected by both Moscow and Damascus.
The talks in Istanbul are also expected to focus on energy issues.
The two countries enjoy strong trade and energy ties, and are expected to look to strengthen those links during Monday's talks.
Ahead of the meeting, Mr Putin's chief foreign policy aide was quoted as saying that the two sides would have a "detailed conversation" on Syria. Yuriy Ushakov also said he hoped that such an "exchange of views should lead, if not to a tie-up of positions, then at least to a better understanding of each other's actions", Russia's Interfax news agency reported.
There is a belief in the Turkish government that Russia is trying to distance itself from Damascus, but there is currently very little evidence to support this claim, the BBC's James Reynolds in Istanbul reports.
Moscow also has arms contracts with the Syrian government worth billions of dollars.
Mr Putin and Mr Erdogan may struggle to find a way of overcoming the simple, profound difference that separates them on the Syria issue, our correspondent adds.
President Putin is also expected to raise his concerns about some of Ankara's recent moves, including its request for Nato to deploy Patriot anti-missile systems on the Turkish border with Syria. Turkey says it is a purely defensive move, but Russia has warned that it will undermine stability in the volatile region.
The Istanbul meeting will be Mr Putin's first venture outside Russia since a visit to Tajikistan in October. Reports in the Russian media have suggested that the 60-year-old keen sportsman is suffering from a bad back.
President Alexander Lukashenko of Belarus, which has close relations with Russia, told Reuters news agency last week that Mr Putin had recently "twisted his spine" playing judo.
In Japan, Prime Minister Yoshihiko Noda was quoted by local media as saying he had postponed a planned visit because "President Putin's health condition is bad".