Recep Tayyip Erdogan and Vladimir Putin, leaders with remarkable similarities who built a new era of cooperation between Turkey and Russia, have suffered a ferocious falling-out that will harm relations for years to come.

The two presidents, who forged a strong friendship in over a decade in power, have traded a slew of insults since Turkey shot down a Russian warplane on November 24 on the Syrian border for alleged repeated violations of its airspace.

Moscow's top intelligence agency has warned that 10 Syrians linked to the Islamic State group have entered Thailand to target Russians, the kingdom's police said Friday.

A leaked letter, marked "top secret" and "urgent" and signed by the deputy head of Thailand's special branch, was widely circulated in local media late Thursday.

The foreign ministers of Turkey and Russia met in Belgrade on Thursday in the first high-level bilateral talks since the downing of a Russian warplane by Turkish jets sparked an unprecedented crisis. 

The meeting between Russia's Sergei Lavrov and Turkey's Mevlut Cavusoglu was held on the sidelines of the ministerial council of the Organization for Security and Cooperation in Europe.

President Recep Tayyip Erdogan on Thursday said Turkey had proof Russia was involved in illegal oil trade with the Islamic State (IS) group in Syria, countering "immoral" Russian allegations that his own family was importing oil from the jihadists.

"We have the proof in our hands. We will reveal it to the world," Erdogan said in a televised address in Ankara.

Russia and Turkey, at loggerheads since Ankara shot down a Russian warplane last week, have suspended talks on their joint TurkStream project to pipe gas to Turkey and southern Europe, Russia's energy minister said Thursday.

"Currently talks are suspended," Energy Minister Alexander Novak said, quoted by RIA Novosti state news agency.

Russian President Vladimir Putin on Thursday vowed Turkey's leadership would be made to regret the downing of one of Russia's warplanes as Moscow announced a halt to talks on a major gas pipeline project.

As Putin fired another salvo in his mounting war of words with NATO member Turkey his Turkish counterpart Recep Tayyip Erdogan declared he had "proof" that Russia was involved in illegal oil trading with the Islamic State (IS) group in Syria.

Russian President Vladimir Putin on Wednesday made a surprise visit to Crimea, inaugurating an undersea cable project to bring a new power supply to the strategic Black Sea peninsula.

Until late November, Crimea's electricity had mostly been supplied by Ukraine. But the territory's population of nearly two million has been plagued by power cuts since Crimea's four main pylons in Ukraine were blown up late last month.

Russia's Foreign Minister Sergei Lavrov said Wednesday he was willing to meet his Turkish counterpart this week for the two countries' first high-level face-to-face talks since Ankara shot down one of Moscow's warplanes.

Lavrov said Turkish Foreign Minister Mevlut Cavusoglu was "insisting" on holding talks on the sidelines of the OSCE Ministerial Council in Belgrade on Thursday and Friday.

U.S. Secretary of State John Kerry said Wednesday that Washington welcomed Russia's involvement in efforts to end the Syrian conflict, where Moscow could play an "extremely constructive" role.

"We welcome Russia's engagement in the Syria process. As long as they're focused on Daesh (Islamic State) and as long they are genuine in wanting to be part of implementing the Geneva (accords), they can be an extremely constructive and important player in reaching a solution," Kerry said after NATO talks in Brussels.

President Recep Tayyip Erdogan on Wednesday vowed Turkey would not retaliate against Russia's "emotional" sanctions imposed after the disputed downing of a Russian war plane.

Russia has halted the sale of tours to Turkey -- a key tourist destination -- and is set to ban the import of Turkish fruit and vegetables as well as threatening other measures.