New York, March 21  The US will close the southern border with Mexico to non-essential travel on Saturday because of the coronavirus pandemic, Secretary of State Mike Pompeo announced.

He said on Friday that the two countries had worked together on the closure plan in order "to limit the spread of the virus" while ensuring "that commerce that supports our economy continues to keep flowing".

On the north, Canada and the US announced their plans on Wednesday to close the border.

Acting Homeland Security Security Chad Wolf said: "Neither of these agreements with Canada or Mexico applies to lawful trade or commerce. Essential commercial activities will not be impacted."

He said that the border would be open to those travelling for medical purposes, to attend educational institutions, for emergency response, public health services and cross-border trade.

Mexico is the biggest trading partner of the US with $614.5 billion in trade last year, overtaking China.

The US depends a lot on Mexico for food imports. Agricultural workers coming in from there under a visa system for them are the backbone of farm labour in California.

Pompeo said that they would not be affected by the new restrictions.

Tens of thousands of people seek asylum at the Mexican border and the border closure will affect them.

Many thousands also cross the border illegally to the US and many of them seek asylum.

From midnight Friday, Wolf said that those arriving without documentation would be immediately returned to Canda, Mexico and other countries.

Trump said that those who are not Mexicans would be returned to their own countries, adding, "why would Mexico take people that aren't from Mexico? We're sending them back" to their own countries.

Wolf said that his department was acting under the orders of the Centers for Disease Control and Prevention (CDC) to not allow in anyone without proper documents.

He said the Department apprehended people from 120 countries, many of which have the coronavirus trying to enter the US.

"Many of these individuals arrive with little or no identity, travel, or medical documentation, making public health risk determinations all but impossible."

ompeo said that Mexico had agreed to stop people coming in from the 28 European countries, China, South Korea who may try to circumvent the ban on their entry to the US by traveling through Mexico.