Smoke rises from a military airport in Ukraine days after Russia invaded in February 2022. A deserter stationed at a top secret base in Russia said nuclear weapons "were fully in place" on the day of the attack. ARIS MESSINIS/AFP via Getty Images

Nuclear weapons at a top-secret base in Russia were "fully in place" and ready to be launched when President Vladimir Putin ordered the invasion into neighboring Ukraine in February 2022, a deserter told the BBC.

The former officer, identified only as Anton to protect his anonymity, told the outlet that he was stationed at a secretive nuclear arms base that was put on full combat alert in the leadup to the attack.

"Before that, we had only exercises. But on the day the war started, the weapons were fully in place," he told the BBC. "We were ready to launch the forces into the sea and air and, in theory, carry out a nuclear strike."

Chillingly, the former officer also said he was given a "criminal order" to inform the troops of specific guidelines for the use of the nukes.

"They said that Ukrainian civilians are combatants and should be destroyed!" he said. "That's a red line for me - it's a war crime. I said I won't spread this propaganda."

Anton said the alert was in place on the first day of the war and his unit was "shut inside the base."

"All we had was Russian state TV," the former officer said. "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."

Three days before Russian troops entered Ukraine, Putin ordered his top military leaders to "transfer the Russian army's deterrence forces to a special mode of combat duty."

Anton said the alert was in effect for more than two weeks before it was canceled. He said he and the other officers at the base were carefully watched to ensure secretive information didn't leak from the base.

"There is a very strict selection process there. Everyone is a professional soldier – no conscripts," he told the BBC.

"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."

Life at the base was highly regimented, he said.

"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," he told the BBC.

"It was my responsibility to ensure the soldiers under me didn't take any phones on to the nuclear base," he told the BBC.

He said Putin controls a substantial and modern nuclear arsenal, rejecting claims from western experts that many of Russia's nuclear missiles date to the Soviet era. Anton said that was 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," he said.