A US homeowner who shot and killed two teenagers, calling them "vermin" after they broke into his house in Minnesota, has been convicted of their murders.

Byron Smith shot unarmed cousins Haile Kifer (18) and Nick Brady (17) a total of nine times as they crept into his Little Falls home on Thanksgiving Day in 2012.

Mr. Smith (65) said it was self-defence and that he had been in fear of his life after previous burglaries in the area.

But prosecutors said Smith had plotted to kill the intruders - waiting with loaded guns in his basement and even parking his vehicle elsewhere to make the property appear unoccupied.

On Tuesday, a jury found Smith guilty of two counts of first-degree murder and two counts of second-degree murder.

The trial heard he had set up an audio-recording device and surveillance cameras to capture the intruders in the act.

In the recording that was played in court, Smith could be heard shooting Brady and saying "You're dead". He then opened fire on a screaming Kifer.

As the audio continued to run, Smith was then heard saying "I see them as vermin".

The killings prompted debate in the State about how much deadly force a householder may reasonably use.