Moscow (Reuters): Dmitry Medvedev, a senior Russian security official who served as Russia's president from 2008 to 2012, warned the United States on Saturday to take Russia's nuclear warnings seriously to avoid World War Three.

Medvedev, who serves as deputy chairman of Russia's powerful security council, told RT broadcaster that top US officials did not want World War Three but for some reason they believe "that the Russians will never cross a certain line." "They are wrong," Medvedev told RT, adding that Moscow believed the current US and European political establishments lacked the "foresight and subtlety of mind" displayed by the late Henry Kissinger.

"If we are talking about the existence of our state, as the president of our country has repeatedly said, your humble servant has said, others have said, of course, we simply will not have any choice," Medvedev said.

The 2-1/2-year-old war in Ukraine is entering what Russian officials say is its most dangerous phase as Russian forces are advancing in eastern Ukraine and the West considers how to shore up Ukraine.

Russia has been signalling for weeks to the West that Moscow will respond if the United States and its allies help Ukraine fire longer-range missiles deep into Russia, while NATO says that North Korea has sent troops to western Russia.

Russian officials say the leaders of the West have failed to heed the signals Moscow has sent over European security and the escalation of the war in Ukraine.

US diplomats say the relationship with Russia is worse than at any time since the depths of the Cold War but that Washington does not seek to escalate the war in Ukraine.