Ukrainian President Volodymyr Zelenskyy says an operation by his country's military in Russia's western region of Kursk is unfolding exactly as Kyiv expected. But a US institute cautions that it is still premature to evaluate achievements in the incursion.

Zelenskyy referred to the cross-border offensive in a video address on Saturday. He said Ukrainian forces are reinforcing their positions and that the foothold of their presence is getting stronger.

But a US think tank, the Institute for the Study of War, said on Sunday that it is still "too early to assess the outcomes and operational significance" of the incursion.

But the institute says the offensive "has already generated theater-wide operational and strategic pressures on Russian forces."

It adds that "subsequent phases of fighting within Russia will likely generate even greater pressures" on President Vladimir Putin and the Russian military.

Meanwhile, Russia denied a media report that the incursion had derailed indirect talks between Moscow and Kyiv on halting strikes on their energy and power infrastructure.

Russia's Foreign Ministry spokesperson Maria Zakharova said on Sunday that there were and are no such direct or indirect negotiations between the two sides on the matter.