From The Russo-Ukraine War: The Return of History, by Serhii Plokhy (W. W. Norton, 2023), Kindle p. 65:
Territorial disputes are a hallmark of imperial disintegration, and the fall of the USSR was no exception. The Russian government had been challenging Ukraine’s territorial integrity even before it became legally independent and left the Soviet Union. The first challenge to the Ukrainian borders came from the democratic government of Russia almost immediately after the Ukrainian parliament declared the country’s independence on August 24, 1991. Two days later the Russian president’s spokesman, Pavel Voshchanov, made a statement on behalf of his superior.
“In the most recent days, state sovereignty has been declared and withdrawal from the USSR announced in a series of Union republics,” read the statement. “In that regard, I have been empowered by the President of the RSFSR to make the following declaration. The Russian Federation casts no doubt on the constitutional right of every state and people to self-determination. But there is the problem of borders, which may prove to be unregulated, a condition admissible only if provision is made for Union relations secured by an appropriate treaty. Should they be abrogated, the RSFSR reserves the right to pose the question of revision of borders.”
The statement was addressed to every Soviet republic that might declare its independence of the Soviet Union. But when Voshchanov was asked by journalists to be more specific, he singled out Ukraine and Kazakhstan. “If those republics enter into a union with Russia, then there is no problem,” he explained. “But if they withdraw, then we must be concerned about the population living there and not forget that those lands were colonized by Russians. Russia will hardly agree to give them away so easily.” Both Ukraine and Kazakhstan had large ethnic Russian minorities, and both republics, Ukraine in its entirety, and Kazakhstan in its northern lands, were eyed as parts of a future Russian state by no less a figure than Aleksandr Solzhenitsyn, whose article advocating the creation of such a state had been published by the major Soviet newspapers the previous year.
Kyiv and Almaty protested, obliging Yeltsin to dissociate himself from Voshchanov’s remarks. The spokesman was portrayed as someone who had got out of control and presented his personal views rather than the policy of Yeltsin’s administration. But Voshchanov had in fact formulated the new policy of the Russian Federation for years to come. Treaties recognizing the borders of Union republics like the one signed between Russia and Ukraine in 1990 applied only if the republics, Ukraine in particular, remained in union with Russia. The understanding of what such union meant would change over time, from Gorbachev’s Soviet Union to Yeltsin’s Commonwealth of Independent States and, eventually, a number of Eurasian projects advanced by Putin. Models and rulers changed, but the basic principle remained the same: Russia’s recognition of the territorial integrity and sovereignty of the post-Soviet states would be conditional on alliance with Moscow.