Ukraine and most international observers have dismissed the annexations as illegal, a position Kyiv reiterated in response to Peskov's remarks.
"Ukraine has its internationally recognized borders," Ukrainian Foreign Ministry spokesman Georgiy Tykhy said.
"It's really laughable to see them referring to their constitution" to justify the annexations, he added.
Russian forces control most of the Donetsk and Luhansk regions but only parts of Zaporizhzhia and Kherson. Moscow also occupies part of Ukraine's northeastern Kharkiv region.
Ukrainian President Volodymyr Zelensky has acknowledged that his army lacks the resources to reclaim all occupied territories by force but has suggested that some land could be returned through diplomacy.
Meanwhile, Russian and U.S. officials met in Istanbul on Thursday to discuss embassy-related issues as part of efforts to stabilize ties.
The talks, held at the U.S. consul general's residence, followed this month's first high-level meeting between the two nuclear powers since Russia launched its full-scale invasion of Ukraine.
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