The Nigerian army has cited a video of US President Donald Trump, in which he says soldiers should respond with force to migrants throwing stones, to justify opening fire on a Shia group this week.

"When they throw rocks... consider it as a rifle," Mr Trump says in the clip.

Nigerian police have arrested 400 members of a Shia Muslim sect after days of deadly protests in the capital.

Amnesty International has criticised Nigeria's army for the killings, saying the Shia protesters were peaceful.

But a spokesman for the Nigerian army says their decision to fire live rounds at protesters in Abuja was justified because they were armed, telling the BBC "this is what [Mr] Trump was talking about".

The army's official Twitter account shared the video adding the caption "please watch and make your deductions".

The clip shows the US president saying, in reference to Central American migrants, "they want to throw rocks at our military, [then] our military fights back".

Nigeria Army spokesman Brig Gen John Agim says the army posted the video in reaction to the rights group's report accusing the army of using weapons against Shia protesters.

The army did not mention the fact that the US embassy in Abuja has urged Nigerian authorities to "take appropriate action to hold accountable those responsible for violations of Nigerian law". (BBC)

The number of Shia Muslim protesters killed by Nigeria's army in the capital, Abuja, in clashes which began at the weekend, has not been independently verified.

The Nigerian army says six protesters have been killed, but the protest group itself says dozens died, and rights group Amnesty International says the true number is 45.