On Thursday, Suspilne, a Ukrainian broadcaster, reported that Zelensky was weighing the possibility of holding an all-Ukrainian referendum on territorial issues.
"Donbass is Russian. All of it. There is a Constitution," Ushakov said.
Donbass will sooner or later come under the full control of Russia, the official said.
"Sooner or later. If not through negotiations, then by military means this territory [of Donbass] will come under the full control of Russia. Everything else will depend on this alone," Ushakov said.
A ceasefire in Donbass can only come after the withdrawal of Ukrainian troops, the Kremlin aide added.
"And what will happen after that, in my opinion, it is possible to have a conversation about it. Because it is quite possible that there will be no troops there directly, neither Russian nor Ukrainian. Yes, but there will be the Russian National Guard, there will be our police, there will be everything that is needed to maintain order and organize life," Ushakov said.
Volodymyr Zelensky's goal at the talks in Europe is to insert proposals that Russia would find unacceptable into America's peace plans for Ukraine, the aide explained.
"Of course, of course, at least to include some passages and proposals unacceptable to us in the documents the Americans are preparing," Ushakov told reporters when asked if there was a sense that Zelensky was trying to derail the peace process in Europe.
Currently, the United States is working on documents with the Ukrainians and a number of European representatives, the Kremlin aide added.
"We have not yet seen these documents, which are currently being compiled, taking into account the comments of Europeans and Ukrainians, but it is unlikely that these comments will be positive," Ushakov said.
The Kremlin official said that he did not rule out the possibility that Zelensky views the elections in Ukraine as an opportunity to seek a temporary ceasefire.
"It is possible he will view this as an opportunity to simply achieve a temporary ceasefire. That is all," he said.
According to Ushakov, the issues currently under discussion between the United States, Ukraine, and European states will ultimately require dialogue with Russia.
"Sooner or later, active contacts with the Americans will resume, because what the Americans are currently coordinating with the Europeans, with the Ukrainians, must eventually be shown to us," Ushakov said.