White House officials have been circulating a proposal that would give U.S. border authorities the extraordinary ability to block U.S. citizens and permanent residents from entering the country from Mexico if they are suspected of being infected with the novel coronavirus, according to two administration officials and a person familiar with the plans.

It is unclear whether the Trump administration has the legal authority to block citizens and permanent residents from returning to their own country, but one official said the administration is weighing a public health emergency declaration that would let the White House keep out potentially infected Americans.

Medical experts have warned the administration that such restrictions would make little difference in controlling the pandemic, because widespread community transmission already is occurring in the United States. The country’s outbreak is the world’s worst, with more than 5 million confirmed cases and wide circulation in many parts of the country, especially in the South and Southwest.