Moscow: Russian President Vladimir Putin raised the spectre of nuclear war with the West on Friday as he defied international condemnation over his decision to send thousands of Russian troops and heavy armour into eastern Ukraine.
Accused by Europe and NATO of launching a full-scale invasion, he boasted to a group of Russian youngsters that "It's best not to mess with us".
In language not heard since the height of the Cold War, he told his audience: "Thank God, I think no one is thinking of unleashing a large-scale conflict with Russia. I want to remind you that Russia is one of the leading nuclear powers."
Mr Putin's comments, made during a visit to a pro-Kremlin youth camp on the banks of a lake outside Moscow, will alarm Western governments. Even during the Cold War, few Kremlin leaders resorted to the direct mentions of Russia's nuclear arsenal.
Mr Putin made his remarks as European leaders were preparing to hold an emergency summit to discuss further sanctions. NATO believes there are more than 1000 regular Russian troops in eastern Ukraine.
The soldiers are believed to be the backbone of a counter-offensive in which pro-Kremlin rebels in eastern Ukraine have seized back swathes of territory from Ukrainian government forces in the past few days, dramatically turning the tide in the four-month conflict.
A battle is now looming in the port city of Mariupol, where Ukrainian forces are braced for a full-scale assault by rebels backed by Kremlin's forces. The escalation in the conflict is the most serious since the pro-Russian uprising began, and has dashed Western hopes that sanctions had forced the Kremlin to abandon its support for the rebels.
"If it looks like a war, sounds like a war and kills like a war, it is a war," Poland's Foreign Minister Radoslaw Sikorski said. "The situation is now out of control," his German counterpart, Frank-Walter Steinmeier, added.
At a summit in the Welsh city of Newport next week, NATO leaders will meet Ukraine's new president, Petro Poroshenko, to show what Mr Rasmussen described as "unwavering support".
Diplomatic sources have said Ukraine will ask for a package of "non-lethal" aid, including thousands of new uniforms, helmets, body armour and communications gear. Ukrainian defence chiefs also want access to sophisticated US and NATO satellite images of Russian troop positions.
Ukraine Prime Minister Arseny Yatseniuk said on Thursday his country would now seek to join NATO, although sources within the organisation said it was out of the question in the foreseeable future. Any such move would oblige NATO to come to Ukraine's immediate defence against Russia.
On Friday, Mr Putin appeared to give the rebels direct encouragement by describing them as the forces of "Novorossiya", or "New Russia".
The rebel advance along the south-eastern coast suggested Mr Putin may be establishing the basis for a more independent eastern Ukraine, or for an overland route from Russia to Crimea, the southern Ukrainian peninsula that Russia annexed by Russia five months ago.
A takeover of Mariupol, an industrial city of 450,000, would go a long way towards helping the separatists gain control over land that would connect Russia to Crimea.
"We plan to take Mariupol," said a commander of the rebels in Novoazovsk, who identified himself by his nickname, Svet. "Now we are fighting for the south-east of Ukraine for Novorossiya."
Journalists who visited Mariupol earlier on Friday saw Ukrainian workers digging trenches with backhoes and building defensive positions in anticipation of an assault. Civilian residents, household belongings piled into their cars, were leaving.
Telegraph, London; New York Times