ISTANBUL/MOSCOW (Reuters) - Standing up to Bashar al-Assad is one thing. Picking a fight with Vladimir Putin would be something else entirely.
By forcing down an airliner flying from Moscow, and publicly accusing Russia of ferrying military equipment to Damascus, Turkey's Prime Minister Tayyip Erdogan has taken what may prove to be the biggest gamble yet in his Syria policy.
The incident risks damaging a carefully nurtured relationship with an irascible Russian superpower at a time when Ankara needs all the friends it can get.
For more than a year the two have kept disagreement over Syria from spilling over into a relationship governed by Turkish need for energy supplies, Russian desire for pipeline routes and mutual security interests across an array of regional hotspots.
"The risk now is that Syria is the wedge that forces apart what had been a growing and dynamic partnership between Moscow and Ankara," said Nikolas Gvosdev, professor of national security studies at the U.S. Naval War College.
"Because the grounding of the plane was done in such a public manner, Putin will see this as a direct challenge," he said of the Russian president.
Russia provides nearly two-thirds of Turkey's gas supplies and often ramps up its exports to the country during frequent cuts in Iranian gas supplies in the winter. Russia is also set to help build Turkey's first nuclear power plant.
Russia also plans to build its 63 billion cubic meter South Stream pipeline through Turkey's waters to feed Europe. The plan raises Turkey's profile as a partner in the project and gives both countries incentives to maintain friendship.
Officials in Turkey's energy ministry say they see little likelihood that those relationships would be harmed over Syria.
"Both countries need each other. This reciprocal need will ensure that the energy sector remains protected," said Turkish energy analyst Haluk Direskeneli.
Improved relations with Russia are particularly valuable for Turkey at a time when its leading role in opposition to Assad has cost it other friendships in its region. Iran, Assad's biggest backer, has become embittered, and a decision to host an Iraqi Sunni fugitive vice president has deeply hurt Ankara's ties with Baghdad.
"Russia and Turkey had been in the middle of a rapprochement unique in the history of Russian-Turkish relations. This seems to have destroyed that, at least for now," said Andrew Kuchins, head of the Russia program at the Centre for Strategic and International Studies in Washington.
Erdogan has said the Syrian Air Airbus A-320, forced to land in Ankara on Wednesday after the Turks received an intelligence tip-off, was carrying Russian-made munitions bound for Syria's defense ministry.
One source close to the Turkish government suggested Turkey could have avoided trouble if it had intercepted the flight in secret and warned the parties involved privately, rather than making strong public statements. Its rhetoric rather than its actions were the source of its problems, the source said.
Two days after the plane was intercepted, the Turkish authorities have yet to give any details on what was in the seized cargo. Russia has received no response to its requests for information, a Russian foreign ministry source said.
Turkey, the only member of the NATO Western military alliance that borders Syria, has felt increasingly isolated over its role as one of the main opponents of President Assad during a 19-month-old uprising that has killed about 30,000 people.
Ankara has provided sanctuary for rebel officers and led calls for international intervention.
The Turkish military has repeatedly fired across the border over the past ten days in response to gunfire and shelling spilling over from Syria and has warned of a more robust response if Syria fails to contain the violence.
It scrambled two fighter planes to the border on Friday after a Syrian military helicopter bombed the Syrian town of Azmarin, right on the frontier.
The stance has won it cheers of approval from the United States, European allies, and NATO - as well as Arab countries ruled by Sunni Muslims who oppose Assad, a member of the Alawite minority sect - but little in the way of concrete support.
"Turkey is quite isolated on Syria. We do get routine expressions of support from NATO, from Washington, but they don't really have much flesh to them," said Faruk Logoglu, vice chairman of the main opposition Republic People's Party (CHP).
Russia, meanwhile, has been Assad's biggest supporter at the United Nations, and sold Syria $1 billion of arms last year. Yet until now, Ankara has managed to avoid allowing Syria to wreck its relationship with Moscow.
"It is a sensitive issue but there are ways of decoupling the Syrian question from the Turkish-Russian bilateral relationship," former Turkish foreign minister Yasar Yakis, one of the founding members of the ruling AK Party, told Reuters.
"Whether we will be able to achieve it is another question."
Moscow supports Assad both to protect a rare ally in the Middle East, and because Putin believes as a matter of principle that Russia should use its clout to prevent the West from interfering in countries' internal affairs.
An arms industry source said Moscow had not stopped its weapons exports to Damascus, despite Western criticism.
Foreign Minister Sergei Lavrov said on June 9 that Russia was fulfilling existing contracts for supplies of air defense systems, for use against external attacks, and is not sending Syria weapons that could be used in the internal conflict.
Turkish and Russian media reports said the cargo seized in Turkey included communications equipment, which would fall under what Moscow insists are legal exports.
A local news website in the city of Tula, around 200 km from Moscow and home to several defense industry firms, said the goods came from a factory there which makes air defense systems and other high precision weapons. A representative from the factory told Reuters it has contracts with Syria, but declined to comment further.
A spokesman for Russia's defense export monopoly said it had no arms on the jet, but individual Russian defense companies can sign contracts directly with clients in certain situations.
"If there was some radio-electronic equipment, one possibility is it was illegal smuggling, which means that Syrian authorities dealt directly with those who produce this equipment and they didn't inform Russian authorities. A second variant is that it was a secret operation by the Russian government," said Alexander Golts, a Moscow-based defense analyst.
He said sales, although legal, may be carried out more secretively in order to avoid the kind of diplomatic pressure Russia attracted after shipments of combat helicopters and ammunition to Syria in two separate incidents earlier this year.
For now, Moscow and Ankara appear to be trying to avoid statements that would escalate the quarrel.
Putin held a meeting about Syria with his advisory Security Council on Friday. A report by the state-owned RIA news agency said deteriorating relations between Turkey and Syria were discussed, but made no mention of the plane incident.
Putin had been expected to visit Turkey at the start of next week. Turkish officials said hours before the plane was grounded that Russia had requested the visit be postponed, citing his heavy work schedule.
Asked if the postponement was linked to the grounding, Putin's spokesman Dmitry Peskov said Putin and Erdogan had discussed a new date by phone on Monday, two days before the incident, and December 3 was likely.
Additional reporting by Peter Apps in Washington, Steve Gutterman and Gleb Bryanski in Moscow; Writing by Nick Tattersall; Editing by Peter Graff