Russia said on Thursday that Ukrainian membership of the U.S.-led NATO military alliance would be unacceptable to Moscow.

"For us, the threat is still the expansion of NATO," Russian Foreign Ministry spokeswoman Maria Zakharova told reporters in Moscow. "NATO's desire to pull Ukraine into its orbit remains a threat to us."

"This country's membership of an aggressive military alliance that has declared Russia to be its target, calling it a security threat in all operational environments, is unacceptable to us."

One of President Vladimir Putin's central conditions for ending the war in Ukraine is a demand that Western leaders pledge that Ukraine would not become a NATO member, Reuters has reported.

At the 2008 Bucharest summit, NATO leaders agreed that Ukraine and Georgia would one day become members. Ukraine in 2019 amended its constitution committing to the path of full membership of NATO and the European Union.

U.S. President Donald Trump has said that previous U.S. support for Ukraine's NATO membership bid was a cause of the war, and has indicated that Ukraine will not get membership.

In the text of a 28-point U.S. proposed peace plan for Ukraine, point 7 said: "Ukraine agrees to enshrine in its constitution that it will not join NATO, and NATO agrees to include in its statutes a provision that Ukraine will not be admitted in the future."