The Russian president issued the chilling warning to a group of journalists as he attempted to explain the reasons behind his country’s actions in recent months.
Maintaining a steely exterior, Mr Putin discussed the potential start of a second Cold War and warned the world is in “grave danger”.
He said: “Major global conflicts have been avoided in the past few decades, due to the geostrategic balance of power, which used to exist.
“The two super-nuclear powers essentially agreed to stop producing both offensive weaponry, as well as defensive weaponry.
“It’s not in my nature to scold someone – but when the United States unilaterally withdrew from the Anti-Ballistic Missile Treaty 1972 they delivered a colossal blow to the entire system of international security.
“That was the first blow when it comes to assessing the strategic balance of power in the world.
“At that time in 2002 I said we will not be developing such systems also, because A) it is very expensive, and B) we aren’t sure yet how they will work for the Americans.”
Mr Putin revealed how Russia has gone on to develop new weapons in response to continuing American developments.
He accused the US of rejecting Russian deals to cease weapon production and insists Russia can pinpoint when the US will build a new missile that will directly threaten his country.
Mr Putin said: “We told them about the reactionary measures we were going to take. And this is what we did. And I assure you – that today, we have had every success in that area.
“I’m not going to list everything, all that matters is we have modernised our military-industrial complex and we continue to develop new generation warfare.
“I’m not even going to mention systems against the missile-defence system.
“No matter what we said to our American partners to curb the production of weaponry they refused to cooperate with us, they rejected out offers, and continue to do their own thing.
“Some things I cannot tell you right now publicly that would be rude of me and whether or not you believe me, we offered real solutions to stop this. They rejected everything we had to offer.”
The US moved nuclear weapons from Turkey to Romanic earlier this year in response to threats from Iran.
But Mr Putin rubbished the American explanation, suggesting the White House lied to Russia over the reasons behind the positioning of their missiles.
He said: “So here we are today and they’ve placed their missile defence system in Romania always saying ‘we must protect ourselves from the Iranian nuclear threat’.
“Where’s the threat? There is no Iranian nuclear threat. You even have an agreement with them and the US was the instigator of this agreement, where we helped. We supported it. But if not for the US then this agreement would not exist which I consider President Obama’s achievement.
“I agree with the agreement, because it eased tensions in the area. So President Obama can put this in his list of achievements. So the Iranian threat does not exist.
“But the missile defence systems are continuing to be positioned. That means we were right when we said that they are lying to us. Their reasons were not genuine, in reference to the ‘Iranian nuclear threat’. Once again, they lied to us.
“So they built this system and now they are being loaded with missiles.”
The Russian president added the US missiles in Romania are being loaded with ‘anti-missiles’ that can penetrate distances of up to 500km.
He added: “But we know that technologies advance. We even know in which year the Americans will accomplish a new missile, which will be able to penetrate distances of up to 1000km, and then even further.
“And from that moment on they will be able to directly threaten Russia’s nuclear potential.
“We know year by year what’s going to happen and they know that we know. It’s only you that they tell tales to, and you spread it to the citizens of your countries.
“Your people, in turn, do not feel a sense of the impending danger – this is what worries me.
“How can you not understand that the world is being pulled in an irreversible direction?”
Furious with the US for feigning ignorance over the nature of their weaponry, Mr Putin added the White House could load up rocket launchers with a nuclear arsenal and no government would even be aware of the change of missiles.
He said: “Those capsules into which ’anti-missiles’ are inserted, as I’ve mentioned, they are sea-based on warships which can carry the Tomahawk subsonic cruise missile system.
“One could deploy it to position in a matter of hours and then what kind of ‘anti-missile’ system is that?
“How do we know what kind of missile is in there? All you have to do is change the programme from non-nuclear to nuclear. That’s all it would take.
“This would happen very quickly, and even the Romanian government itself, won’t know what’s going on.
“Do you think they let the Romanians call any shots? Nobody is going to know what is being done- not the Romanians and the Polish won’t either. Do you think I’m not familiar with their strategies?
“From what I can see, we are in grave danger.”
Continuing to speak to journalists at the St Petersburg Economic Forum, the video of the summit has only recently surfaced online, Mr Putin said the US has discussed developing ballistic missiles with the Kremlin.
He said: “We have conversations once with our American partners – where they said they’d like to develop ballistic missiles, but without a nuclear warhead, and we said ‘do you actually understand what that might entail? So you’re going to have missiles launching from submarines, or ground territories- this is a ballistic missiles. How do we know whether or not it has a nuclear warhead?’
“Can you even imagine what kind of scenario you can create?
“But as far as I am aware, they did not go through with developing these weapons- they have paused for now. But the other one they continue to implement.
“I don’t know how this is all going to end. What I do know is that we will need to defend ourselves and I even know how they will package this ‘Russian aggression’ again. But this is simply our response to your actions.
“Is it not obvious that I must guarantee the safety of our people?”
Mr Putin added: “I think this is gravely dangerous. I not only think that, I am assured of it.”
The publication of the video comes as it was claimed the US had upgraded its threat level from DEFCON 5, the least severe, to DEFCON 3, meaning President Obama could mobilise troops in as little as 15 minutes.