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.

After an ultimatum from US President Donald Trump, the Kremlin responded with threats and referenced the Soviet-era nuclear strike system. In response, the White House deployed nuclear submarines closer to Russia.