Russian President Vladimir Putin swiped back at US President Donald Trump for calling Russia a "paper tiger".
Mr Trump said he thought Ukraine could win back all territory from Russia during his address to the United Nations last week, and labelled Russia a "paper tiger". He repeated the line this week.
"A paper tiger. What follows then? Go and deal with this paper tiger," Mr Putin said at the Valdai Discussion Group in the Black Sea resort of Sochi on Thursday, local time.
"Well, if we are fighting with the entire NATO bloc, we are moving, advancing, and we feel confident, and we are a 'paper tiger', then what is NATO itself?"
The Russian president also warned the United States that if it supplied Tomahawk missiles to Ukraine it would trigger a dangerous new escalation.
"It is impossible to use Tomahawks without the direct participation of American military personnel," he said.
"This will mean a completely new, qualitatively new stage of escalation, including in relations between Russia and the United States."
The US has so far not announced any decision on supplying Tomahawks to Ukraine.
Putin teases NATO countries to take Russia on
Mr Putin said Russian forces were advancing along the entire front in Ukraine and that almost all of the US-led NATO alliance was now fighting against Russia.
NATO members, he said, were providing Ukraine with intelligence, weapons and training, and whipping up what he cast as hysteria about alleged plans of Russia to attack a NATO member, which he dismissed as "impossible to believe".
"If anyone still has a desire to compete with us in the military sphere, as we say, feel free, let them try," Mr Putin said.
"Russia's countermeasures will not be long in coming."
Western European leaders and Ukraine cast the war as an imperial-style land grab and have repeatedly vowed to defeat Russian forces.
They argue that unless Russia is defeated, Mr Putin will risk an attack on a NATO member.
"We simply cannot ignore what is happening. We have no right to do so for reasons of our own security. I repeat, our defence and safety. Therefore, we are closely monitoring the escalating militarisation of Europe," the Russian president said.
"I just want to say: Cool down, sleep calmly, and take care of your own problems. Just take a look at what's happening on the streets of European cities."
He added that he believes Ukraine's armed forces had a grave lack of manpower and desertions, while Russia had enough soldiers.
He suggested that Kyiv should negotiate an end to the war.
Russian-US relations at 'lowest point in memory'
In his speech to the discussion forum on Thursday, Mr Putin said Russia and the US have "many points of disagreement".
"Our views on many global issues clash. This is normal for such major powers," he said.
"In fact, naturally, the most important thing is how to resolve these disagreements, and to what extent they can be resolved peacefully."
The Russian president said he feels the Trump administration "states its interests and desires directly" and "bluntly, but without any unnecessary hypocrisy".
He said that is a "rational approach" for the US to take but added "Russia also reserves the right to be guided by its national interests".
Asked about what he and the US president discussed when they met in Alaska in August, he said "practically nothing".
The summit ended with many believing the Russian president outplayed his US counterpart.
Mr Putin said the talks did not achieve much at all and said it was largely superficial.
"We discussed restoring Russian-American relations, which are not just at an impasse, but at their lowest point in memory," he said.
ABC/Reuters
