Russian President Vladimir Putin is using nuclear "blackmail" to keep the international community from interfering in his Ukraine invasion, the head of the Nobel prize-winning group ICAN said.

"This is one of the scariest moments really when it comes to nuclear weapons," Beatrice Fihn, who leads the International Campaign to Abolish Nuclear Weapons, told AFP in an interview Tuesday.

The 40-year-old Swede, who has spearheaded the group's global efforts to ban the weapons of mass destruction since 2013, said she had never in her lifetime seen the nuclear threat level so high.

"It is incredibly worrying and overwhelming." 

Just days after Russia launched its full-scale invasion of its pro-Western neighbour on February 24, Putin ordered his country's nuclear forces to be put on high alert, sparking global alarm.

Addressing the US Congress on Tuesday, Avril Haines, US Director of National Intelligence, described Putin's move as "extremely unusual".

"We have not seen a public announcement from the Russians regarding a heightened nuclear alert status since the 1960s," she pointed out.

Fihn described the move as "extremely dangerous".

"Not only is this meant to instil fear in the whole world; it's also meant to scare anyone from helping in Ukraine." 

- 'Everyone is terrified' -

Countries during the Cold War argued that large nuclear arsenals served as deterrents, helping avoid conflict. Now Moscow was using its arsenal to enable conflict, she said.

"Russia is using it to blackmail almost, to be able to invade Ukraine, and nobody can interfere."

The nuclear threat "is now being used in an extremely malicious and evil way, to... enable an illegal invasion of a country that doesn't have nuclear weapons."

Would Putin actually use nuclear weapons? Fihn stressed she still did not think it was likely.

But "it is not ruled out", she said. "We are starting to worry that it might happen."

But even if there is no plan to actually use such weapons, with tensions soaring "misunderstandings can escalate quickly and we could stumble into nuclear use by accident", she warned.

Fihn said she had received numerous messages from people asking how to speak to their children about the threat.

"Everyone is terrified right now," she said, acknowledging that the situation was getting to her too.

"I spent the last decade talking about what happens when a nuclear weapon is used, what happens to bodies, what happens to cities," she said. 

"I am finding it very difficult to talk about it now." 

- 'Wake-up call' -

But Fihn hopes the current crisis will serve as a wake-up call that will push countries towards nuclear disarmament.

"If we survive this, we're not going to be so lucky all the time," she said.

"We cannot let countries do this to other countries anymore, (just) because they have nuclear weapons."

ICAN won the 2017 Nobel Peace Price for its key role in drafting the Treaty on the Prohibition of Nuclear Weapons, which took effect a year ago.

Fifty-nine countries have ratified the treaty, and more have signed it.

Fihn pointed out that the treaty bans the kinds of nuclear threats being made by Russia, which is not a party to it. Nor are any of the states known to possess nuclear weapons.

But the current crisis has sparked growing interest in the treaty, she said.

"I feel like there's an opening now that we can really start working towards nuclear disarmament."

Once the conflict ends, she said, Russia should not be permitted to maintain its current nuclear arsenal.

"They're going to have to do something... in order to be able to be let back into the international community again, and nuclear disarmament should be that."

IAEA says loses contact with Chernobyl

The Chernobyl nuclear power plant is no longer transmitting data to the UN's atomic watchdog, the agency said Tuesday, as it voiced concern for staff working under Russian guard at the Ukrainian facility.

On February 24, Russia invaded Ukraine and seized the defunct Chernobyl plant, site of a 1986 disaster that killed hundreds and spread radioactive contamination west across Europe.

International Atomic Energy Agency chief Rafael Grossi "indicated that remote data transmission from safeguards monitoring systems installed at the Chornobyl NPP had been lost", the agency said in a statement.

"The Agency is looking into the status of safeguards monitoring systems in other locations in Ukraine and will provide further information soon," it said.

The IAEA uses the term "safeguards" to describe technical measures it applies to nuclear material and activities, with the objective of deterring the spread of nuclear weapons through early detection of the misuse of such material.

More than 200 technical staff and guards remain trapped at the site, working 13 days straight since the Russian takeover.

The situation for the staff "was worsening" at the site, the IAEA said, citing the Ukrainian nuclear regulator.

The defunct plant sits inside an exclusion zone that houses decommissioned reactors as well as radioactive waste facilities. 

More than 2,000 staff still work at the plant as it requires constant management to prevent another nuclear disaster.

The UN agency called on Russia to allow workers to rotate because rest and regular shifts were crucial to the site's safety.

"I'm deeply concerned about the difficult and stressful situation facing staff at the Chornobyl nuclear power plant and the potential risks this entails for nuclear safety," said Grossi.

"I call on the forces in effective control of the site to urgently facilitate the safe rotation of personnel there."

With remote data transmission cut off and the Ukrainian regulator only able to contact the plant by email, Grossi reiterated his offer to travel to the site or elsewhere to secure "the commitment to the safety and security" of Ukraine's power plants from all parties.

Russia also attacked and seized Europe's largest atomic power plant, Zaporizhzhia, last week, drawing accusations of "nuclear terror" from Kyiv.

Zaporizhzhia alone has six reactors of a more modern, safer design than the one that melted down at Chernobyl.

The IAEA said two of those were still operating, the plant's personnel were working in shifts and radiation levels remained stable.