Mr Johnson spoke out on the day Mr Corbyn accused Theresa May of "pandering" to Donald Trump during a speech in which the Labour leader insisted he was "not a pacifist".
Speaking during a Conservative Party campaign visit to an indoor market in Newport, south Wales, Mr Johnson said: "I am genuinely worried about some of the things Jeremy Corbyn stands for and believes in.
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"This is a time of great uncertainty in the world.
I am genuinely worried about some of the things Jeremy Corbyn stands for and believes in
"We have Russia causing all sorts of trouble in eastern Europe and elsewhere, not least Syria.
"We have a global threat from terrorism and we have to be firm, we have to support Nato and we have to have a very robust response to the threat of terrorism.
"Jeremy Corbyn is a guy who has campaigned all his life to weaken the UK's defences.
"He thinks Nato is nonsense. I think he is still the chairman of the Campaign for Nuclear Disarmament and I find it genuinely alarming that just last month when we sent the advanced forward presence to shore up our friends in the Baltic and UK troops went to Estonia, he said that was an escalation of tensions.
"What this country needs is a strong, clear and robust approach to our defences and I am afraid Jeremy Corbyn's approach is chaotic and offers the reverse."