WASHINGTON (AFP) - A 20-year-old man armed with an assault rifle has been arrested at a Walmart store in the midwestern US state of Missouri and charged with making a "terrorist threat," police said.

Dmitriy Andreychenko, who was wearing body armour and fatigues, was detained by an armed off-duty firefighter who held him until police arrived, police said.

No shots were fired and no injuries were reported in the incident on Thursday (Aug 8) in the city of Springfield, which has a population of around 160,000.

Springfield police said Andreychenko has been charged with making a terrorist threat.

"At this time, the investigation is on-going and we are working to determine his motives," police said in a statement.

The incident came less than a week after an avowed white supremacist killed 22 people in an attack at a Walmart in El Paso, Texas, and another gunman shot dead nine people in Dayton, Ohio.