At least nine migrants drowned and dozens of others were rescued after attempting to cross the Rio Grande river from Mexico into the United States, officials said Saturday.

US Customs and Border Patrol (CBP) said in a statement that the drownings occurred on Thursday, when a large group attempted to cross the Rio Grande near Eagle Pass, Texas.

Thirty-seven people were rescued but the bodies of nine migrants were found -- three by Mexico authorities and six by US agents.

A total of 53 migrants were detained on the US side and Mexican officials on their side of the river picked up 39, CBP said in a statement.

No information was provided about the ages or nationalities of the migrants.

"The search continues for other possible victims," CBP added.

A CBP official told the Washington Post that the mass drowning appeared to be the worst in years along the Rio Grande.

Eagle Pass lies within the CBP's Del Rio region, where nearly 50,000 migrants were arrested last month, according to government data.

The city's fire chief, Manuel Mello, told the New York Times that the migrants had been swept away by strong currents about a mile south of the International Bridge that links Eagle Pass with Piedras Negras, Mexico.

Mello noted that drownings had become frequent in the area, with as many as one per day.

Recent mass migrant deaths have highlighted the dangerous journey hundreds of thousands take each year trying to reach the United States from Mexico.

In June, over 50 people died after being abandoned in a scorching hot trailer in San Antonio, Texas.