US special envoy Steve Witkoff has arrived in Moscow and was greeted by Russia’s investment envoy Kirill Dmitriev, a source told Reuters on Wednesday, after the Kremlin said it did not rule out Witkoff meeting President Vladimir Putin.

The comments come as Trump's deadline for Russia to agree a ceasefire with Ukraine or face sanctions edges closer.

Russia said on Monday that everyone should be "very, very careful" about nuclear rhetoric, responding to a statement by U.S. President Donald Trump that he had ordered a repositioning of U.S. nuclear submarines.