The group was taken into custody during a battalion formation at Camp Pendleton, a major Marine base in California about 65 miles (90 kilometers) north of the Mexican border.

"The 16 Marines were arrested for alleged involvement in various illegal activities ranging from human smuggling to drug-related offenses. Information gained from a previous human smuggling investigation precipitated the arrests," the military said in a statement.

It added that eight other Marines "were taken aside to be questioned on their involvement in alleged drug offenses unrelated to today's arrests."

Marine spokeswoman Major Kendra Motz said the charges were a "mixed bag" and not all of the arrested Marines were accused of human trafficking.

Thursday's arrests came after Marines Byron Darnell Law II and David Javier Salazar-Quintero were taken into custody earlier in the month for allegedly smuggling undocumented Mexican migrants into the US.

The pair were charged after three Mexican nationals who were prepared to pay $8,000 "to be smuggled into the United States" were found in the back of their vehicle about six miles over the border, a court document said.

Motz said the case against Law and Salazar-Quintero "identified the allegations" against the 16 Marines arrested Thursday, who also belonged to the same unit at Camp Pendleton as the two men.

"There could potentially be a connection" between the cases, Motz said, declining to give further details as both investigations remain ongoing.

Similar instances of US military members helping migrants enter the US were reported in 2014, 2017 and last year.

The military said the Marines arrested Thursday were not part of the military deployment ordered to the US-Mexico border as part of President Donald Trump's plan to stem the flow of undocumented migrants.