Ukraine's border guards have stopped over 20,000 men from leaving the country since the beginning of Russia's full-scale invasion, according to an official.

"In total, border guards have detained about 14,600 people trying to leave Ukraine illegally since February 24 last year," border guard spokesperson Andriy Demchenko told a TV channel on Tuesday, adding that about 6,200 men were caught with fake exit permits. 

Demchenko said many of these men — aged 18-60 — sought to evade military conscription. 

In the face of mounting counteroffensive challenges, Kyiv is trying to curtail the corruption enabling these men to evade military service. 

Under a January law, those evading the draft or deserting can now face 5 to 12 years of imprisonment.

Kyiv is also considering extraditing over 650,000 Ukrainian males aged 18-64 who have sought refuge across the European Union.

Ivan Ishchenko, 30, who fled Ukraine after seeing a month of combat, told AFP news agency that he went to war thinking he "was a superhero." 

"But all heroism ends when people see (war) with their own eyes and realize that they don't belong there," Ishchenko said. 

He said he eventually paid thousands of dollars and risked prison to leave the front.

The demand for black market exemption papers in Ukraine has risen exponentially, with costs surpassing €10,000 ($10,721) following recent crackdowns.