“The polite reply here was that this is U.N. business and not for the E.U. to entertain,” Mr. Pierini said. “The idea is dead in the water.”

Underscoring the degree of Turkish concern about Russia’s military involvement in Syria, which seemed to expand this week when Russian officials in Moscow suggested Russian “volunteer” fighters would be sent to Syria, officials in Ankara, the Turkish capital, predicted a new influx of refugees. Speaking to the Turkish daily Hurriyet, Deputy Prime Minister Numan Kurtulmus said that up to one million more Syrians could arrive in Turkey, which is already straining under the weight of nearly two million Syrian refugees.

At first Russia suggested that its priority was fighting the Sunni militants of the Islamic State, an aim shared by the United States, which is leading an international coalition that for more than a year has waged an air campaign against the group in Iraq and Syria. But Russia has deployed military equipment, such as ground-to-air missiles and interceptor jets, that has no use against militant groups that do not have an air force. This made clear that Russia’s priority is to buck up Mr. Assad, and it has raised concerns that if a no-fly zone or safe zone were established, as Turkey has pushed for, it could be challenged by Russia.

“The Russian presence has changed the entire parameters in Syria, including a safe zone,” said Mensur Akgun, director of the Global Political Trends Center, a research organization in Turkey. “No one will dare confront Russia.”

The Russians pushed back on Tuesday against objections that their military actions in Syria are not targeting the Islamic State. Maria V. Zakharova, a Foreign Ministry spokeswoman in Moscow, said news media organizations that report such objections are part of what she called an anti-Russian misinformation campaign.

Syrian fighters are in a continual state of churn, she said, alternatively forming alliances and fighting one another, and Russia will not make distinctions between the various groups she described as terrorists. “If they talk like a terrorist, if they fight like a terrorist, if they act like a terrorist, they are a terrorist,” she said, paraphrasing what Foreign Minister Sergey V. Lavrov said at the outset of the campaign last week.

The new tensions between Russia and Turkey over Syria have highlighted a deeply complex relationship between the two countries. In some ways Turkey’s relationship with Russia is similar to its relationship with Iran, the Syrian government’s most important regional ally. In its dealings with each country, Turkey is able to separate sharp differences over issues like the war in Syria with deepening economic ties, not to mention a robust tourism industry between the two countries.