Mexico City: A passenger bus plunged into a ravine in northwest Mexico's Nayarit state early Thursday morning, leaving at least 18 people dead, local authorities said.

Among the dead were three minors, with 22 others injured, local media said, citing Jorge Benito Rodriguez, Secretary of Security and Citizen Protection of Nayarit.

According to local reports, the bus was traveling from Mexico City to Tijuana on the border with the United States. It fell into a ravine about 50 meters deep along the north bypass of Tepic.

Rescue efforts were "extremely" complicated due to the location, said Rodriguez, adding that state police and firefighters, in coordination with the Mexican Red Cross and other agencies, were on the scene.

The cause of the accident remains unknown, he said.