On the day of Russia’s full-scale invasion of Ukraine in February 2022, Anton says the nuclear weapons base he was serving at was put on full combat alert.

“Before that, we had only exercises. But on the day the war started, the weapons were fully in place,” says the former officer in the Russian nuclear forces. “We were ready to launch the forces into the sea and air and, in theory, carry out a nuclear strike.”

I met Anton in a secret location outside Russia. For his own protection, the BBC will not reveal where. We have also changed his name and are not showing his face.

Anton was an officer at a top-secret nuclear weapons facility in Russia.

He has shown us documents confirming his unit, rank and base.

The BBC is unable to independently verify all the events he described, although they do chime with Russian statements at the time.

Three days after troops poured over Ukraine’s borders, Vladimir Putin announced that Russia’s nuclear deterrence forces had been ordered into a “special mode of combat service”.

Anton says that combat alert was in place on day one of the war and claims his unit was “shut inside the base”.

“All we had was Russian state TV,” says the former officer, “I didn’t really know what it all meant. I automatically carried out my duties. We weren’t fighting in the war, we were just guarding the nuclear weapons.”

The state of alert was cancelled, he adds, after two to three weeks.

Anton’s testimony offers an insight into the top-secret inner workings of the nuclear forces in Russia. It is extremely rare for service members to talk to journalists.

“There is a very strict selection process there. Everyone is a professional soldier – no conscripts,” he explains.

“There are constant checks and lie-detector tests for everyone. The pay is much higher, and the troops aren’t sent to war. They’re there to either repel, or carry out, a nuclear strike.”

The former officer says life was tightly controlled.

“It was my responsibility to ensure the soldiers under me didn’t take any phones on to the nuclear base,” he explains.

“It’s a closed society, there are no strangers there. If you want your parents to visit, you need to submit a request to the FSB Security Service three months in advance.”

Anton was part of the base’s security unit – a rapid-reaction force that guarded the nuclear weapons.

“We had constant training exercises. Our reaction time was two minutes,” he says, with a hint of pride.

Russia has around 4,380 operational nuclear warheads, according to the Federation of American Scientists, but only 1,700 are “deployed” or ready for use. All the Nato member states combined possess a similar number.

There are also concerns about whether Putin could choose to deploy “non-strategic”, often called tactical, nuclear weapons. These are smaller missiles that generally don’t cause widespread radioactive fallout.

Their use would nevertheless lead to a dangerous escalation in the war.

The Kremlin has been doing all it can to test the West’s nerves.

Only last week Putin ratified changes to the nuclear doctrine – the official rules dictating how and when Russia can launch nuclear weapons.

The doctrine now says Russia can launch if it comes under “massive attack” from conventional missiles by a non-nuclear state but “with the participation or support of a nuclear state”.

Russian officials say the updated doctrine “effectively eliminates” the possibility of its defeat on the battlefield.

But is Russia’s nuclear arsenal fully functional?

Some Western experts have suggested its weapons mostly date from the Soviet era, and might not even work.

The former nuclear forces officer rejected that opinion as a “very simplified view from so-called experts”.

“There might be some old-fashioned types of weapons in some areas, but the country has an enormous nuclear arsenal, a huge amount of warheads, including constant combat patrol on land, sea and air.”

Russia’s nuclear weapons were fully operational and battle-ready, he maintained. “The work to maintain the nuclear weapons is carried out constantly, it never stops even for one minute.”

Shortly after the full-scale war began, Anton said he was given what he describes as a “criminal order” – to hold lectures with his troops using very specific written guidelines.

“They said that Ukrainian civilians are combatants and should be destroyed!” he exclaims. “That’s a red line for me – it’s a war crime. I said I won’t spread this propaganda.”

Senior officers reprimanded Anton by transferring him to a regular assault brigade in another part of the country. He was told he would be sent to war.

These units are often sent in to battle as the “first wave” and a number of Russian deserters have told the BBC that “troublemakers” who object to the war have been used as “cannon fodder”.

The Russian embassy in London did not respond to a request for comment.

Before he could be sent to the front line, Anton signed a statement refusing to take part in the war and a criminal case was opened against him. He showed us documents confirming his transfer to the assault brigade and details of the criminal case.

He then decided to flee the country with the help of a volunteer organisation for deserters.

“If I had run away from the nuclear forces base, then the local FSB Security Service would’ve reacted decisively and I probably wouldn’t have been able to leave the country,” he said.

But he believes that, because he had been transferred to an ordinary assault brigade, the system of top-level security clearance failed.

Anton said he wanted the world to know that many Russian soldiers were against the war.

The volunteer organisation that helps deserters, “Idite Lesom” [‘Go by the Forest’, in English, or ‘Get Lost’] has told the BBC that the number of deserters seeking help has risen to 350 a month.

The risks to those fleeing are growing, too. At least one deserter has been killed after fleeing abroad, and there have been several cases of men being forcibly returned to Russia and put on trial.

Although Anton has left Russia, he says security services are still looking for him there: “I take precautions here, I work off the books and I don’t show up in any official systems.”

He says he has stopped speaking to his friends at the nuclear base because he could put them in danger: “They must take lie-detector tests, and any contact with me could lead to a criminal case.”

But he is under no illusion about the risk he is himself in by helping other soldiers to flee.

“I understand the more I do that, the higher the chances they could try and kill me.”