BEIRUT, Lebanon — Turkey’s leader denounced President Bashar al-Assad of Syria on Wednesday as a terrorist mass murderer with no place in that country’s postwar future, scrapping a softened approach that Turkish officials had taken toward Mr. Assad in recent years.
The statement by President Recep Tayyip Erdogan of Turkey came as Mr. Assad seemed more confident than ever that he has won the war and will remain Syria’s leader for the foreseeable future. It also came against the backdrop of maneuvering by many powers — most notably Russia and Iran, Mr. Assad’s most important allies — to influence the outcome of a devastating conflict that has reshaped Middle East politics.
One of the first leaders in the region to condemn Mr. Assad when the conflict began in 2011, Mr. Erdogan had in recent months signaled a willingness to accept Mr. Assad’s political longevity.
The Turkish leader’s shift on Wednesday was a reminder of their hostility, coming as Mr. Assad has demonstrated greater swagger over his grip from military gains over the past year, largely with Russia’s help.
In a new sign of his confidence, Mr. Assad even allowed a modest medical evacuation of civilians on Wednesday from one of the last rebel enclaves in the country, near Damascus.
Mr. Erdogan appeared to be reminding Russia that it cannot dictate Syria’s future alone, especially on issues sensitive to Turkey, most notably those involving Syria’s Kurdish groups, which Turkey sees as enemies.
Russia on Tuesday said that representatives of a semiautonomous Kurdish area in northeastern Syria would be allowed to take part in talks that Russia is hosting next month — an inclusion opposed by Turkey.
“Assad, I am saying this loud and clear, is a terrorist who spreads state terrorism,” Mr. Erdogan said at a joint news conference with the Tunisian president, Beji Caid Essebsi, in Tunis. “Would the Syrian people like to see someone like this stay in charge?”
In remarks quoted by Turkish news agencies, Mr. Erdogan also said: “It is absolutely impossible to move ahead with Assad in Syria. For what? How could we embrace the future with the president of a Syria who killed close to one million of its citizens?”
Furious over the insult, Syria’s Foreign Ministry called Mr. Erdogan a terrorist supporter who bore “prime responsibility for the bloodshed in Syria.”
The conflict has killed hundreds of thousands — there are no reliable figures — along with upending roughly half of Syria’s prewar population and contributing to a migration crisis that has reverberated around the world. At least 5.4 million Syrians are refugees and more than six million are internally displaced, the United Nations says.
Russia and Iran have always backed Mr. Assad, while Turkey supports some Syrian rebel groups. Despite their differences, the three nations have been collaborating on diplomacy aimed at ending the war.
All three also have been jockeying for position in the country’s post-conflict future, even as their efforts to end the fighting have proved only partly successful.
Mr. Erdogan’s statement appeared to signal more of a tough negotiating stance than a rupture with Russia, which has been enjoying an improved relationship with Turkey, a NATO member. Even as Mr. Erdogan spoke, his government in Ankara was finalizing a $2.5 billion deal to purchase Russian S-400 missile systems.
It is possible the Russians welcomed Mr. Erdogan’s tough line toward Mr. Assad, because they want to play a leading role in any peace deal. That means delivering an often recalcitrant Mr. Assad to negotiations.
A main issue between Russia and Turkey has involved Syria’s Kurds. Mr. Erdogan has made clear lately that preventing them from maintaining a semiautonomous area bordering Turkey has become a higher priority than toppling Mr. Assad.
But Moscow has been eager to include Kurdish groups in peace talks. It has won greater inclusion for them in the United Nations-backed talks in Geneva — though not through the separate Kurdish delegation that the Kurds wanted — and now has invited numerous Kurdish representatives to Sochi, the southern Russia resort town where talks that Moscow calls a Syrian “national dialogue” will supposedly be held in late January.
“It hasn’t worked well,” Andrew J. Tabler, a Syria expert at the Washington Institute for Near East Policy, said of Turkey’s effort on the Kurds. Insistence on blunting Kurdish power in Syria, he said, “takes the limelight in Turkey.”
A previous attempt to convene talks in Sochi, in November, failed when Turkey withdrew over objections to Kurdish participation.
The planned January meeting has also been widely snubbed by Mr. Assad’s Syrian opponents. Forty rebel groups declared Tuesday that they would not take part.
Before the Arab revolts of 2011, relations between Syria and Turkey were neighborly, along a border that stretches more than 500 miles. But six months into the Syrian uprising — which began with political protests met with a harsh security crackdown — Mr. Erdogan broke with Mr. Assad, saying he must step down.
Mr. Erdogan then went on to finance Syrian rebel groups and later allowed foreign recruits to the Islamic State and other jihadist militant groups to stream through Turkey into Syria.
But the Syrian war has taken a toll on Turkey, which is housing more than three million refugees and has suffered deadly attacks by the Islamic State and Kurdish groups.
Soon after Russia began its air campaign on behalf of Mr. Assad’s government in 2015, Turkey shot down a Russian warplane. Russia retaliated with sanctions that were devastating for Turkish trade and tourism.
Turkey’s antipathy toward the Kurds, oddly, is partly responsible for the reconciliation in Turkish-Russian relations and a Turkish shift away from insistence that Mr. Assad must go.
As Russian air power severely weakened Syria’s rebel forces, Turkey was willing to temper its support for them in exchange for Russia’s assent to a Turkish sphere of influence in northern Syria, where Turkey could block Kurdish expansion.
Mr. Erdogan’s condemnation of Mr. Assad on Wednesday came as the Syrian leader appeared to allow a humanitarian breakthrough, albeit a small one, in the besieged Syrian rebel-held enclave of Eastern Ghouta, home to about 400,000 people and the only major rebel stronghold near Damascus.
The International Committee of the Red Cross in Syria said on Wednesday that after protracted negotiations, it had been able to start medical evacuations from Eastern Ghouta.
The enclave has been targeted by Mr. Assad’s forces, and the United Nations has pleaded for his government to allow for the evacuation of around 500 patients, including children with cancer.