Now, nearly 20,000 men have fled Ukraine since the beginning of the war to avoid being drafted. A further 21,000 have tried to leave, but were caught by the Ukrainian authorities. That's according to a BBC-I investigation. Since Russia's invasion, Ukraine has banned men of fighting age from leaving the country unless they have a valid exemption. Zana Bezpiatchuk has more on how some men are managing to escape with the help of corrupt officials..

Stopped by border police, the men are ordered onto their knees. For Ukrainians without a military exemption, crossing the border illegally is the most popular way of escaping the draft. Some get caught. But for much of this war, dozens of men have managed to flee this way every day. Using freedom of information requests, BBC I has established that the most common route out is over Ukraine's border with Moldova. In a refugee center in Moldova, we meet Eric, a musician who is hoping to go to the US to be reunited with his family. He's arrived from Ukraine on foot.

I just looked at the map. I heard rumors about where to go, took a risk, and it worked out.

Eric says he has a serious health condition, but wasn't granted a medical exemption, commonly known as a white ticket.

I tried to process the documents for six months, but no one allowed me to do it. My patience ran out and I decided to live this way.

We heard that some men are paying substantial bribes to obtain a white ticket. To find out more, we asked the local journalist to pretend to be a man who wants to leave Ukraine. A quick search reveals at least six groups offering help on the messaging app Telegram. Our undercover journalist contacts one of them and is soon offered a white ticket. Officials at these centers work closely with military doctors to decide who can get a medical exemption.

They send me an example of this white ticket. For me, this document looks real. Probably they know somebody at that office. So this is corruption.

The BBC showed its findings to a senior politician from the ruling party.

We are.

Doing our best to reduce the number of corruption cases. Those who try to avoid mobilization are about one to five %. They are definitely not critical to the defense of Ukraine.

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Ukraine.

Doesn't release official figures on the size of its army, but says it has over 800,000 troops. As this conflict turns into a war of iteration, Ukraine will need all the soldiers it can get.