As Ukraine's military and citizens battle Russia's advancing troops, the country has opened a new front in the fighting - using tech expertise to rally Silicon Valley's support and undermine the enemy. Digital minister Mykhailo Fedorov is leading the charge, but some of his tactics are proving divisive.

From his underground shelter in a secret location in Kyiv, Ukraine's youngest cabinet minister is waging a digital war on Russia.

Using his preferred weapon - social media - Mykhailo Fedorov has been urging chief executives of big businesses to cut ties with Moscow. He's also taken the unprecedented move of setting up a volunteer "IT Army of Ukraine" to launch cyber-attacks against "the enemy".

At only 31, Fedorov has shaped his government role around his lifestyle - he lives through and on his mobile phone.

Before the war, his main goal was to create a "state in a smartphone", where 100% of government services would be offered online. Now that project is on hold, with every muscle strained on the digital war effort.

He has piled pressure on multinational companies to boycott Russia.

Apple, Google, Meta, Twitter, YouTube, Microsoft, Sony, Oracle... no tech giant has missed out on an official government letter.

Fedorov then posts his letters on social media so the world can see, plus some of the replies.

It's impossible to say whether this has influenced the companies' actions, but most have changed their policy towards Russia in subsequent days - either stopping products being sold there, like Apple, or halting operations.

Saturday's announcement from PayPal that it was suspending services in Russia appeared on Fedorov's Twitter feed before it was reported in the media. So too did news that Samsung and Nvidia are stopping all business with Russia, something he publicly called for on his social feeds.

One tweet from Fedorov to Elon Musk soon after the invasion began brought quick results. Within 48 hours the billionaire tech mogul had adjusted his constellation of Starlink satellites and sent a lorry-load of internet-ready terminals to Ukraine.

The service is a potential lifeline for the government if internet and telecommunication networks are damaged or destroyed, though Musk has since warned that the satellite dishes could become a target for Russian missiles and should be used with care.

Fedorov has more than half a million followers in total across Twitter, Facebook, Instagram and Telegram and uses them all to get his message across.

"Each platform is very important to us now and we are using every opportunity to attract large companies to this horror happening now in Ukraine. We are trying to bring the truth to the Russians and make them protest against the war," he told the BBC via email.

He speaks mostly in Ukrainian online, but since the crisis unfolded he's switched to English on Twitter, where he is having the most impact.

"Twitter has become an efficient tool that we are using to counter Russian military aggression. It's our smart and peaceful tool to destroy Russian economy," he says.

Technology researcher and author Stephanie Hare says she is not surprised Fedorov is having success.

"[He] is 31 years old. He gets it," she says.

Using persuasion and propaganda is a time-honoured tactic of warfare. But since social media companies entered the equation in the 2000s "they have changed the calculus due to the speed and breadth with which people can disseminate their messages".

Fedorov's spokeswoman told me his young team was constantly coming up with new ideas, which the ministry then tries to implement at speed. Last week, Kyiv announced it would issue non-fungible tokens (NFTs) to fund its military. But some others have been controversial.

Fedorov is urging crypto-currency exchanges to freeze the accounts of all Russian citizens, for example - an idea that many, including the CEO of the Binance exchange, says would "fly in the face" of the reason crypto exists.

And the ministry's launch of an "IT Army of Ukraine", including thousands of volunteer hackers from around the world - its Telegram group now has 270,000 members - has caused some unease.

"Tech is the best solution against tanks," Fedorov told the BBC. "The IT Army is directed against the digital and online resources of Russian and Belarusian business corporations, banks, and state web portals. We have shut down the operation of the web portal of the Russian public services, the exchange, websites of Tass, Kommersant, Fontanka, and other top media in Russia."

So far the hacking seems to be mostly low-level cyber vandalism but Fedorov's team is also explicitly calling for attacks on railway and power grid networks which, if successful and disruptive enough, could cause harm to civilians. It makes some in the cyber-security world anxious.

"It's really important to be careful in this realm," says Suzanne Spalding from the Centre for Strategic and International Studies. "If we get into destructive attacks on critical infrastructure being carried out by citizens I think we begin to run into the kinds of fog-of-war, misattribution, potential cascading impacts that weren't anticipated. We might see retaliation from one side for something a citizen has done and things can escalate fast."

On Friday Ukraine's deputy chairman of the State Service of Special Communications, which works closely with Fedorov's department, defended the decision to rally hackers against Russia.

He said he welcomed illegal cyber-attacks on Russia from all groups, including the Anonymous hacking collective, because "the world order changed on 24 February" when the invasion started.

Hacking is also being carried out against Ukraine by people sympathetic to Russia, but currently Russia seems to be coming off worse. Its crack military hackers appear, so far, to have not played a major role, for reasons that are unclear.