By Laura Gottesdiener and Lizbeth Diaz CIUDAD JUAREZ, Mexico (Reuters) - Mexican authorities have begun constructing giant tent shelters in the city of Ciudad Juarez to prepare for a possible influx of Mexicans deported under U.

The shelters in Ciudad Juarez will have the capacity to house thousands of people, a municipal official has said.

A secret tunnel discovered last week on the U.S.-Mexico border will be sealed by Mexican authorities, an army official in Ciudad Juarez said Saturday.

President Donald Trump's promises of mass deportations, which could bring batches of new arrivals fresh off the border bridges into Juárez, has Mexican law enforcement preparing to keep watch for potential trouble.

The Mexican government plans to establish nine reception areas for deportees in Mexico's six northern border states over the coming weeks.

General Jose Lemus, commander of Ciudad Juarez's military garrison, said the tunnel "must have taken a long time" to build, suggesting "it could have been one or two years".

All along the 2,000-mile border, business leaders, border agents, migrants and their advocates are bracing for change in U.S. policies on trade and immigration.

With President Trump back in office, Mexico's President Sheinbaum braces for strained relations. U.S. public support for hardline policies targeting Mexico is on the rise, and looming energy reforms and trade negotiations add to the challenges.

Immigration: Migrant encounters at El Paso border at lowest levels in years Juárez is stuck between the politics of the U.S. and Mexico ... specifically Ciudad Juárez," Gilberto Loya, public ...

In the state of Chihuahua, which includes Ciudad Juarez, authorities reported a ... authorities to detain them and transfer them to Mexico City for later deportation. The National Institute ...

The Mexican government is building nine shelters in border cities to receive deportees. It has said that it would also use the existing facilities in Tijuana, Ciudad Juarez and Matamoros to take in migrants whose appointments to request asylum in the US were cancelled on Trump's inauguration day.

Discovered on January 10 by US and Mexican security agencies, the tunnel measures approximately 300 meters (1,000 feet) in length on the Mexican side and is equipped with lighting, ventilation and is