Informed sources, with whom the journalists spoke, said that it will not be easy to do this, but the Western countries still have certain methods that will help to detect the first signs of preparations for nuclear war.

In its publication, Politico refers to an interview with several former and current US officials.

They say the U.S. is stepping up intelligence, trying to understand exactly what actions by the Russian military -- or messages they exchange -- could indicate Putin's decision to launch a nuclear strike.

And they fear that such a warning will not come to Washington too late.

The authors recall the recent statements of Russian President Putin and Deputy Secretary of the Security Council Dmitry Medvedev, in which they hinted at the possibility of using all possible means to protect "territorial integrity".

They refer to experts who are convinced that Moscow can go precisely to the use of nuclear charges of relatively low power.

What is the difficulty

The main difficulty in trying to predict the used tactical (but not strategic) nuclear charge is that similar warheads are capable of carrying most Russian aircraft and conventional missiles.

"This means that if Putin or his commanders do not want the world to know about their intentions in advance, the United States may not understand at what moment Russia changed the conventional charge to atomic bombs," the publication said.

In order to track possible signs of preparations for nuclear war, the American intelligence services study all available information - from commercially available satellite images to data obtained by reconnaissance aircraft or on the Internet.

The US pays special attention to Kaliningrad, a Russian enclave where Russia has installed supersonic missiles and systems that can be used for both conventional and nuclear strikes.

Politico writes that several American BoeingRC-135 electronic reconnaissance aircraft have been circling the city for the past week.

Over the past few years, Russia has upgraded its missile depots in Kaliningrad, raising concerns about a possible stockpile of nuclear weapons in the area.

Nuclear arsenal of Russia

According to one source, Russia has 23 types of weapons capable of carrying both conventional and nuclear warheads, many of which are already in use in Ukraine, such as the Iskander missiles.

According to open data, Russia has more than 1,900 tactical nuclear warheads, which are also called non-strategic nuclear warheads.

"This is everything: from cruise missiles to nuclear torpedoes, gravity bombs and medium-range ballistic missiles," the source of the publication says.

Intelligence services, according to Politico, are confident that Russia will not risk a full-scale nuclear war by launching a massive attack on Ukraine or NATO countries.

"They will never use strategic nuclear weapons.

They will never launch an intercontinental ballistic missile and will not deliver a bomber (Tu-95) with warheads of the megaton class.

What they will do is use melee weapons.

They have warheads, which we call micronuclear charges, with a capacity of tens to hundreds of tons," a US government official told the publication.

Russia is also known to have a lower-power nuclear weapon for use on the battlefield that is equal to or superior to the bombs dropped on Japan.

"American military commanders and intelligence experts hope that the first sign of Russia's intention to use nuclear weapons in Ukraine will not be a nuclear mushroom," the publication said.

"Subtle" signs

Western special services can understand that we are talking about the intention to use weapons of mass destruction, based on not very obvious signs.

For example, if certain units are sent to the battlefield, are all connections except those capable of taking part in a nuclear attack withdrawn.

"We can think - yes, they usually act a little wrong.

This part they send into battle, and the rest withdraw.

They usually don't do that.

It's amazing," the interlocutor of the publication describes one of the possible scenarios.

Other Politico sources are convinced that the "nuclear preparation" of the Russian leadership, on the contrary, may be deliberately open - so that Western countries understand what it is all about and make concessions.

Nuclear cooperation