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."