Jens Stoltenberg, secretary general of the North Atlantic Treaty Organization (NATO), said Wednesday that Russia should withdraw all troops from war-torn Eastern Ukraine, where armed conflict between Moscow-backed separatists and Ukrainians has been ongoing for more than five years.

"NATO states very clearly that Russia has a special responsibility to withdraw all their troops, all their officers," Stoltenberg said while visiting Odesa, a port city on the Black Sea in Southern Ukraine.

"We welcome all efforts to reduce tensions, to withdraw forces and to make sure that we have a peaceful solution to the conflict," he said. "But we know there is a long way to go because there are still cease-fire violations."

While technically at a stalemate, the war in Eastern Ukraine is an immense source of conflict. In 2015, pro-Russian groups protested in the Donbass region in Ukraine, specifically in the self-declared Donetsk and Luhansk People's Republics (DPR and LPR) areas, following the Ukrainian revolution and the overthrow of the government in 2014, as well as Russia's illegal annexation of Crimea in Southern Ukraine the same year. The protests escalated into armed conflict between fighters backed by Moscow and the Ukrainian army.

More than 13,000 people are said to have died to date, including thousands of civilians in the region. More than 1.4 million Ukrainians have been displaced and nearly 1 million have fled the country due to the unrest.

On Tuesday, both sides reportedly pulled back in the DPR and LPR region, as this is a pre-condition for an in-person meeting between Russian President Vladimir Putin and Ukrainian President Volodymyr Zelensky.

The disengagement of troops is seen as the final step before peace talks can begin with Ukraine, Russia, France and Germany, with the intention of ending the violence.

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Zelensky, who was elected in April, made it a priority to end the conflict in Donbass.

However, the president has been met with resistance and protests from Ukrainians who feel the move is an obvious concession to Putin, and would open the door for Moscow to continue to encroach on the country.

Zelensky on Wednesday said the rebel-held regions in Donbass should be reintegrated, and there needs to be "reconciliation and healing the wounds."

The Ukrainian government “has to find a solution that would be supported by an absolute majority," he said. “Society needs to be aware and needs to accept the terms of the reintegration. The lack of a common vision will paralyze our movement.”

Putin in Hungary speculated on whether or not Zelensky could make good on his promise to reintegrate the rebel-held territory.

"[Zelensky] doesn’t look like a Ukrainian nationalist, and it’s hard for me to say if he can manage them,” Putin said. “The question is if he can deliver what he said in public and what he considers necessary to do.”

“I don’t know if he did the right thing as president and commander-in-chief to go to the line of contact to try to persuade the people who refuse to obey his orders,” Putin said, referring to Tuesday when Zelenksy went to the Donbass region to confront protestors who wanted to stop the pullback of troops.

“It’s not such a difficult thing to separate the conflicting sides in two villages, but it has dragged on for years and it’s now clear why -- because the nationalists don’t want to leave and don’t want to let the Ukrainian troops out," Putin said, adding that he was ready for a four-way meeting -- only if it's "well-prepared" and will "produce specific results that will help the settlement."

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"NATO's doors remain open," Stoltenberg said, per the AFP, referring to Ukraine's desire to join NATO.

"Sometimes you get the impression that whether Ukraine should be a member of NATO or not is for Russia to decide," Stoltenberg said.

"Russia doesn't have a say," he added.

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NATO condemns and doesn't recognize Russia's "illegal and illegitimate annexation of Crimea," the organization said.

The Associated Press contributed to this report.