(CNN) Turkish President Recep Tayyip Erdogan has reignited a war of words with the president of neighboring Syria, calling him a "terrorist" who has no place in negotiations over the country's future.

The insults directed to Syrian President Bashar al-Assad came as aid workers evacuated the first of 29 people who had been trapped in the besieged Syrian region of Eastern Ghouta.

Previously, Erdogan had said that around 500 people -- including 170 women and children -- required "urgent humanitarian aid," according to Turkey's TRT and Russia's Sputnik news agencies.

Assad had repeatedly refused aid into the enclave and evacuations out of Eastern Ghouta. It's one of Syria's last rebel strongholds, but it has been under siege by regime forces for more than four years.

On Wednesday, Erdogan said it was "absolutely impossible" to move ahead on Syrian peace talks while Assad remains in power, according to Turkey's state-run news agency Anadolu.

Erdogan made the comments after a closed-door meeting with Tunisian President Beji Caid Essebsi in Tunis where he's on an official visit.

Trading barbs

Erdogan called Assad, who has grimly clung to power during the protracted conflict that has wracked his nation for the past seven years, "a terrorist involved in state terrorism." He asked how anyone could "embrace the future with the president of a Syria who killed close to one million of its citizens."

Syria's Foreign Ministry shot back at the remarks, accusing Erdogan of being "responsible" for spilling the blood of the Syrian people, according to a statement released by Syrian state-run news agency SANA.

"The entry of the Turkish forces to the Syrian territory reveals one image of the Turkey's support to takfiri terrorism," SANA said.

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The statement slammed what it characterized as Erdogan's "paranoia and illusions of the past," which "made him forget that his old empire has vanished and that the free people of the world have the choice to make their national decisions and defend their sovereignty and they will not allow Erdogan to interfere in their affairs."

The US has also said a peaceful solution in Syria is not possible while Assad remains in power. In an opinion piece for the New York Times, US Secretary of State Rex Tillerson said, while the US was "on guard against Russian aggression," the US was willing to work with Russia where "mutual interests intersect."

"Nowhere is that more evident than in Syria," he said.

"Now that President Vladimir Putin has committed to the United Nations-backed Geneva political process for providing a new future for Syria, we expect Russia to follow through. We are confident that the fulfillment of these talks will produce a Syria that is free of Bashar al-Assad and his family," he said.

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Uncertain future

Russia, Iran and Turkey have been working to bring together warring factions in Syria and end hostilities that have dragged on for seven years.

At the end of January, Moscow will host talks involving "all segments of Syrian society" -- including Kurdish representatives, which Turkey opposes -- at an international meeting at the Black Sea resort of Sochi.

Last month, Russian President Vladimir Putin met with Assad for the first since 2015 in the same Russian city to discuss the future of Syria. At the time, Putin said Assad had expressed his commitment to finding a peaceful solution and new elections.

Putin and his government have been one of the chief supporters of the Assad regime, both militarily and in helping to negotiate ceasefires in the country's long-running civil war.

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Evacuations from Eastern Ghouta

On Wednesday, aid agencies began evacuating the first patients who've been trapped in Eastern Ghouta, part of four de-escalation zones negotiated by the Russian, Iranian and Turkish governments in May.

Assad's government has accused rebel groups there of using civilians as human shields, and has refused to allow access to the embattled suburb.

Earlier this week, Erdogan was quoted by Turkish and Russian state media as Turkish and Russian chiefs of staff would work together to carry out the evacuation of hundreds of people who in need of urgent aid, TRT reported.

Since the war began in 2011, an estimated 400,000 Syrians have been killed, according to the United Nations. Assad has ruled Syria as President since July 2000.

The ongoing violence against civilians has been condemned by the Arab League, the European Union, the United States and other countries.

As of March 2017, more than 5 million Syrians have fled the country and 6.3 million people are displaced internally.