President Recep Tayyip Erdogan and Russian counterpart Vladimir Putin inaugurated a new gas pipeline linking their countries at a ceremony in Istanbul on Wednesday with tensions over Libya and Syria also on the agenda.
Erdogan described the Turkstream pipelines -- which will deliver Russian gas to Turkey and Europe via the Black Sea -- as a "project of historic importance" for relations between their countries.
Putin arrived late on Tuesday after paying a surprise visit to Syria -- his first to Damascus since the war began -- at a moment of acute uncertainty in the Middle East following the assassination of top Iranian general Qasem Soleimani by the United States.
TurkStream and the Nord Stream pipelines under the Baltic allow Russia to increase gas supplies to Europe without having to rely on Ukraine.
But Moscow's increasing domination of European energy markets has worried the United States, which last month sanctioned firms working on TurkStream and the almost-completed Nord Stream 2.
The ceremony in Istanbul reflected a dramatic improvement in ties between Russia and Turkey, who appeared on the verge of war less than five years ago after Turkey shot down a Russian jet.
They have established a regular dialogue over the Syrian conflict, despite being on opposing sides, but now find their relations tested again in Libya.
Last week, Turkey sent its first troops to help defend the UN-backed Tripoli government, which is under siege from strongman Khalifa Haftar.
Erdogan has underlined that the deployment remains small for now: only 35 soldiers and limited to training and coordination roles, according to comments carried in the Hurriyet newspaper.
He has criticised the presence of 2,500 Russian mercenaries supporting Haftar -- claims denied by Moscow.
Russia so far seems unfazed by the Turkish deployment in Libya, said Mariana Belenkaia, of the Carnegie Centre in Moscow. "The two countries will likely be tempted to share the Libyan burden," she said.
Syria remains a potential powder-keg for Erdogan and Putin's relationship.
Syrian government forces -- backed by Russia -- have ramped up bombardment of the last rebel strongholds in Idlib province in recent weeks, sending hundreds of thousands fleeing towards the Turkish border.
Erdogan has called for a truce in Idlib, following previous temporary halts to the fighting brokered with Russia in late 2018 and updated in August last year.
"Russia's demands are very simple," said Yury Barmin of the Moscow Policy Group think tank. "Turkey must do more to eliminate terrorist cells in Idlib. The discussions will be around this idea." Improved ties between the two countries have been facilitated by a number of major energy and defence deals.
Russia is building Turkey's first nuclear plant and last year delivered the S-400 missile defence system, to the consternation of Turkey's NATO allies.
Putin earned goodwill in Turkey after his quick support for Erdogan following an attempted coup in July 2016.
The two men have developed a "strong personal relationship", according to Jana Jabbour of Sciences Po university in Paris, who adds that "their economic and energy plans are interdependent".
The TurkStream project, which was temporarily halted during a frosty patch in Russia-Turkey relations, includes two parallel pipelines of more than 900 kilometres (550 miles).
The pipeline links Anapa in Russia to Kiyikoy in northwestern Turkey and has already begun deliveries to Bulgaria. It is being extended towards Serbia, Hungary and Austria.
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