Does karma have effects, only if they are done physically? Or also mentally?

I know Theravada Buddhism has 5 precepts: - killing other beings - stealing, or taking by force what is not given - no sexual relations other than your married soulmate - lying and incorrect speech - consuming intoxicating substances These are physical actions and give bad karma. What if I have negative thoughts in my mind? Would it have the same repercussions as physical actions, or would it be less severe? Thank you.