At least six people were killed and dozens injured after a cargo train carrying US-bound migrants derailed in southern Mexico, officials said.

According to local media reports, ambulances were unable to reach the scene of the accident in the southern state of Tabasco because of the difficult terrain.

Luis Felipe Puente, national emergency service coordinator, told local television four people had been confirmed dead in Sunday's accident and another 35 people were injured, with 16 of those in a serious condition.

The train, known as “La Bestia" or "The Beast", carries Mexican and Central American migrants with many sitting on top of the freight cars after paying smugglers upwards of $100.

The train derailed on a stretch of tracks alongside a river and the site of the accident is only accessible by boat, a public security official from the municipality of Huimanguillo told AFP.

The official said between 250 and 300 migrants were aboard.

The Tabasco state civil protection agency said the train derailed at about 3am local time (8am GMT) and that rescuers were using hydraulic tools to cut through the metal to find survivors. The site is far away from any roads in the area, it added.

A photograph broadcast by Milenio television showed freight cars lying on their side with the wheels detached from the bottom. The tracks are seen in a wooded area and covered with plants.

Source: Al Jazeera and agencies