The Kremlin complained on Monday that Turkey's incursion into northern Syria was "not exactly" compatible with Syrian territorial integrity, and Ankara should ensure its actions were proportionate.
The Kremlin was commenting as Russia-backed Syrian forces deployed deep inside Kurdish-held territory south of the Turkish frontier, less than 24 hours after Washington announced a full withdrawal.
Washington's Kurdish former allies said they invited in the government troops as an emergency step to help fend off an assault by Turkey, launched last week after President Donald Trump moved his troops aside in what the Kurds call a betrayal.
The move was a major boost for Syrian President Bashar al-Assad, Russia's closest Middle East ally, offering him an opportunity to retake territory in the world's deadliest current war.
But it also makes Moscow's balancing act in the eight-year-old conflict war more delicate, as it will put Syrian government troops, who are backed by Russian airpower, in close proximity to the Turkish army and its proxies.
That raises the risk of a clash between two Russian allies and of Moscow's own forces being sucked into some kind of incident with NATO member Turkey, to which it has drawn steadily closer diplomatically.
Russia's Ministry of Defence said media reports sourced to Turkish officials that Moscow had no objections to Turkey including the town of Kobani in its operation were wrong.
"During talks with Turkish colleagues questions about extending Operation Peace Spring, the operation by Turkey's armed forces, to the Syrian town of Kobani were not discussed," it said.
The ministry did not identify the Turkish officials, but President Tayyip Erdogan said on Sunday that Turkey's incursion would stretch from Kobani in the west to Hasaka in the east, going some 30 km (19 miles) into Syrian territory.
On Monday Erdogan said he did not think any problems would arise in Kobani, which had been held by the Kurdish-led Syrian Democratic Forces, after the Syrian army deployed along the border, adding that Russia's Vladimir Putin had shown a "positive approach."
Yuri Ushakov, a senior foreign policy adviser to Russian President Vladimir Putin, said Moscow wanted Turkey to ensure its military incursion into northern Syria was proportionate.
"The main thing is that the Turks act in a way that is proportionate with the situation and that their actions do not harm the most important thing - efforts to get a political settlement in Syria. That is the main thing for us," said Ushakov.
Asked earlier if Turkey's actions squared with Moscow's desire for Syria's territorial integrity to be restored and respected, he said: "Not exactly."
Ushakov, speaking in Riyadh during a visit to Saudi Arabia by Putin, added that Russia planned to "do something" without specifying what that might be.
Dmitry Peskov, Putin's spokesman, said Russia did not want to entertain the possibility of a clash in Syria between Russian and Turkish forces. "We wouldn't even like to think about that scenario," Peskov told reporters.
Peskov said Moscow had already warned all sides in the Syrian conflict to avoid any action that could escalate tensions in the area or damage a fragile political process.
Turkish President Tayyip Erdogan said earlier on Monday that he did not envisage any problems would emerge in Syria's Kobani after the Syrian army deploys along the border.
Erdogan spoke of what he called Putin's "positive approach" to Turkey's actions.
The Kremlin has said previously it is sympathetic to Ankara's need to address security concerns in northern Syria. But Peskov declined to comment on Monday when asked if Moscow felt it was time for Turkey to end its operation inside Syria.
He said Moscow was in regular contact with Ankara, including at a military level. Hours later, Russian Defence Minister Sergei Shoigiu spoke to his Turkish counterpart.
Valery Gerasimov, the head of Russian's military general staff, on Monday also conducted phone negotiations with U.S. Army General Mark Milley, the chairman of the Joint Chiefs of Staff, Russian news agencies reported.