By Ted Hesson, Idrees Ali and Jonathan Landay

WASHINGTON (Reuters) - The Pentagon will send roughly 500 troops to the U.S.-Mexico border to assist federal border agents amid the coronavirus pandemic, three U.S. officials told Reuters.

The sources said the Pentagon approved a request by the U.S. Department of Homeland Security (DHS).

The United States already maintains an average of 5,000 troops at the southwest border to support Border Patrol by performing non-law enforcement duties. The latest deployment will bolster those ranks as border agents grapple with possible exposure to COVID-19, the disease cause by the virus.

Mexico declared a health emergency on Monday and issued stricter rules aimed at containing the fast-spreading coronavirus after its number of cases surged past 1,000 and the death toll rose sharply.

One of the U.S. officials - all of whom requested anonymity to discuss the decision - said the Trump administration worries the pandemic could further depress Mexico’s already troubled economy and encourage illegal immigration from that country to the United States.

DHS did not immediately respond to a request for comment.

The move came as the administration of U.S. President Donald Trump shelved a plan to send troops to the border with Canada, U.S. officials told Reuters.

Canadian Deputy Prime Minister Chrystia Freeland slammed the proposal last week, calling it "an entirely unnecessary step" that would damage bilateral relations between the two nations, which have long maintained an undefended border.

The Trump administration has stepped up its response to the coronavirus outbreak this month as infections have spread across the country. For the first time on Tuesday, the United States recorded nearly 700 new deaths in a single day. The country now has a total of over 3,800 deaths and over 185,000 cases.

The United States closed its northern and southern borders to tourist and recreational travel to limit the spread of the new coronavirus on March 20. At the same time, the Trump administration began to use a health-focused statute to swiftly return migrants caught trying to cross U.S. borders illegally.

The rapid removals also apply to unaccompanied minors from Central America with exceptions on a case-by-case, U.S. Customs and Border Protection said on Monday. The minors previously had been transferred into the custody of the Department of Health and Human Services in accordance with a federal law to protect victims of human trafficking.

(Reporting by Ted Hesson; Idrees Ali and Jonathan Landay in Washington; Editing by David Gregorio and Lisa Shumaker)