Boris Johnson can be seen hurling a dummy grenade as he watches Ukrainian troops train during a trip to a British Army base in North Yorkshire.
The PM donned full camouflage gear as he took part in the training exercise this week, which saw him being shown how to use weapons.
Mr Johnson tweeted: "This week I visited Ukrainian troops being trained by British Armed Forces in North Yorkshire.
"The UK is committed to doing all we can to help Ukraine continue to repel Russian aggression."
He posted a video showing him in action, attending the training session.
"I just want you to know that the people of the United Kingdom support the people of Ukraine and support you in your fight," he said in the video.
"And I am absolutely convinced that you can win and that you will win."
In the video Mr Johnson explains that he has been meeting some of the 400 Ukrainian troops being trained at the base.
He spoke about the UK's "huge commitment" to train Ukraine's armed forces.
"We want to train about 10,000 of them over the next four months," he said.
"That comes on top of the investment we're making in supporting Ukrainians with weaponry, 6900 anti tank weapons, 120 armoured vehicles and much much else besides including now the multiple launch rocket systems that together the Americans, Germans and others we're providing really are starting to make a difference."
Mr Johnson said he hopes to help starting to push back on Putin's evil attack on Ukraine.
He said he knows that in the end Ukrainian people are going to succeed and is proud of the role the UK has been able to play so far.
"But in the end this is thanks to the bravery and heroism of these Ukrainian troops."
This comes after the head of Britain's foreign intelligence service said Russia 's President, Vladimir Putin, has suffered an 'epic fail' in Ukraine.
Richard Moore, the chief of the UK Secret Intelligence Service (MI6) has said that Putin's forces may have to pause the advances on Ukraine in the coming weeks.
