MEXICO CITY — Around 400 migrants expelled from the United States have camped out in a plaza in the dangerous Mexican border city of Reynosa, the aid group Doctors Without Borders said Thursday.

The group said some migrants have disappeared and others are living in poor conditions at the plaza in Reynosa, which is plagued by drug cartels.

The plaza is close to the border bridge where most of the migrants were expelled under pandemic-related powers known as “Title 42.” Many are sleeping in a small kiosk at the plaza, without water supplies or health services, the group said.

“We have reports of people disappearing day and night at the square, which is very worrisome,” said Jose Antonio Silva, the group’s coordinator in Reynosa.

The group said that the numbers of returned or deported migrants are increasing and that the improvised camp could turn into a more longstanding camp like one in the neighboring city of Matamoros that was dismantled a couple of months ago.

The group said Mexico has not established enough shelters to handle returned migrants.

Title 42 is named for a section of an obscure public health law the Trump administration invoked a year ago. Many migrants are expelled to Mexico under the pandemic-era rule that denies them a chance to seek humanitarian protection.

President Joe Biden has kept Title 42 in place as he designs what he promises will be “a humane asylum system.” Citizens of Mexico, Guatemala, Honduras and El Salvador are usually sent back to Mexico, while other nationalities are held in the U.S. to be flown home without a chance at asylum.

Reynosa, a city of 700,000 people, is where many migrants are returned after being expelled from Texas’ Rio Grande Valley, the busiest corridor for illegal crossings. The U.S. Border Patrol has said the vast majority of migrants are expelled to Mexico after less than two hours in the United States to limit the spread of the coronavirus, which means many arrive in Reynosa when it is dark.

In normal times, migrants are returned to Mexico under bilateral agreements that limit deportations to daylight hours and the largest crossings. But under pandemic authority, Mexicans and citizens of Guatemala, El Salvador and Honduras can be expelled to Mexico throughout the night and in smaller towns.

Tamaulipas, that state that includes Reynosa, is among five Mexican states that the U.S. State Department says American citizens shouldn’t visit. A U.S. travel advisory says heavily armed criminal groups patrol Reynosa in marked and unmarked vehicles.