Russian President Vladimir Putin said on Friday that in his view the whole of Ukraine was "ours" and cautioned that advancing Russian forces could take the Ukrainian city of Sumy as part of a bid to carve out a buffer zone along the border.

Ukraine's foreign minister denounced the statements as evidence of Russian "disdain" for U.S. peace efforts and said Moscow was bent on seizing more territory and killing more Ukrainians.

Russia currently controls about a fifth of Ukraine, including Crimea, more than 99% of the Luhansk region, over 70% of the Donetsk, Zaporizhzhia and Kherson regions, and fragments of the Kharkiv, Sumy and Dnipropetrovsk regions.