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.
Despite their differences over Syria, the two countries enjoy strong trade and energy ties, and are expected to look to strengthen those links during Monday's talks.
Meanwhile, reports that Syria's military is moving chemical weapons have drawn a denial from Damascus and a warning from the US.
US Secretary of State Hillary Clinton called it a "red line" for the United States. "I am not going to telegraph in any specifics what we would do in the event of credible evidence that the Assad regime has resorted to using chemical weapons... suffice to say, we are certainly planning to take action if that eventuality were to occur," she said during a visit to Prague.
A Syrian foreign ministry was quoted on state television as saying Syria would "never, under any circumstances, use chemical weapons against its own people, if such weapons exist".
Fighting has been fierce around Damascus in recent days as both sides battle for control of the Syrian capital.
Egypt's authorities ordered an Egypt Air plane en route to Damascus to turn back to Cairo on Monday after being warned of a "bad security situation" around the airport. Many international flights were disrupted at the end of last week because of clashes close to the airport.
The British-based Syrian Observatory for Human Rights has reported that an air strike in the north-eastern border town of Ras al-Ain had killed at least 12 people and wounded more than 30.
Ahead of the meeting between the Russian and Turkish leaders, 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".
But Mr Putin's spokesman Dmitry Peskov has sought to play down the speculation, saying rumours about his boss's health had been "blown out of proportion".