Russia and the United States agreed to resume high-level military contacts, in a major step of rapprochement between the world's top nuclear powers at Ukraine talks in Abu Dhabi on Thursday.

Nighttime shelling by Ukraine inflicted "serious damage" in the Russian city of Belgorod, near the border, the region's governor said early on Friday.

"The work was substantive and productive, focused on concrete steps and practical solutions," Rustem Umerov, the head of Ukraine's National Security and Defence Council, wrote on X.

Without the New START treaty, which caps the number of deployed nuclear warheads at 1,550 on each side, there will be no limits on the U.S. and Russian arsenals.

The last remaining nuclear arms pact between Russia and the United States has expired, removing caps on their atomic arsenals for the first time in over 50 years

Security experts have voiced concern that the treaty's lapse could pave the way for a new arms race in which China would also be a major player.