The world's top two nuclear nations said Thursday they were resuming high-level military contacts, an agreement reached at two-day talks in Abu Dhabi aimed at resolving the Ukraine war.

"There is an understanding, and this was also discussed in Abu Dhabi, that both sides will act responsibly and will recognise the need to start negotiations on this issue as soon as possible," spokesman Dmitry Peskov said of nuclear issues.

The New START treaty -- the last nuclear agreement between Moscow and Washington -- expired on Thursday, triggering fears of a global arms race.

US President Donald Trump's administration repeatedly criticised New START and said a new agreement was necessary, but insisting that it also include China.