MEXICO CITY and LONDON -- At least 16 people were killed and 36 others were injured in a bus crash in central Mexico on Tuesday, authorities said.

A passenger bus carrying locals and migrants -- mostly from Venezuela -- collided with a trailer truck on the Cuacnopalan-Oaxaca highway near the border between the Mexican states of Oaxaca and Puebla early Tuesday morning, according to the Puebla state government.

The reason for the crash was unknown.

There were 52 people on board the bus, including 10 from Venezuela. The migrants were traveling to an appointment with the United States Customs and Border Protection to legally enter the country, according to Mexico's National Institute of Migration.

Among the dead were 15 Mexican citizens and one Venezuelan national. Nine Venezuelans were among the injured, three of whom remain hospitalized in Tehuacan, the National Institute of Migration said.

ABC News' Ellie Kaufman contributed to this report.