Donald Trump and his top military commanders announced a new operation to combat "Mexican drug cartels" and other Central and South American narcotics organisations – saying he fears "losing ground" to such groups.

"We will defend our country regardless of the cost," Joint Chiefs of Staff Gen. Mark Milley said during a White House briefing. "You will not get past jump street.

"We're at war with COVID-19, we're at war with terrorists and we're at war with the drug cartels," the president said. "You will not penetrate this country."

He and Defence Secretary Mark Esper walked through a number of troops movements and deployments to support a new operation.

But his timeline did not align neatly with the White House's and Mr Trump's on calling the Covid-19 outbreak a major threat.

Mr Esper said he began to look at the Pentagon's "inventory" for conducting a Central and South American operation "months ago."

But Mr Trump at that time was saying the deadly virus is "like the flu" and would quickly be over inside the United States.