President Donald Trump said on Thursday that he plans to sign an order next week that could lead to the large-scale detention of migrants crossing the southern border and bar anyone caught crossing illegally from claiming asylum - two legally dubious proposals that mark his latest election-season barrage against illegal immigration.

Mr Trump also said he had told the US military mobilising at the southwest border that if US troops face rock-throwing migrants, they should react as though the rocks were "rifles."

"This is an invasion," Mr Trump declared, as he has previously on a subject that has been shown to resonate strongly with his base of Republican supporters.

He made his comments at the White House in a rambling, campaign-style speech that was billed as a response to caravans of migrants traveling slowly by foot toward the US border.

But Mr Trump offered few details on how exactly he planned to overhaul an asylum system he claimed was plagued by "endemic abuse" that he said "makes a mockery of our immigration system."