This is the moment two men jump off a tugboat as it sinks in one of the world’s largest rivers.

Footage filmed by shocked onlookers shows the moment the vessel is swallowed by the waters of the Rio Negro – a tributary of the Amazon River and the largest blackwater river in the world.

Blackwater rivers get their name from the fact that the water flows through forested or swampy land, with the decaying vegetation causing a dark acidic water that sometimes resembles tea. Most of the key blackwater rivers are in the southern United States and the Amazon Basin.

The incident took place off Ponta Negra beach in the west of the Brazilian city of Manaus on the afternoon of Friday, 25th June.

One amateur video shot at the scene shows how two men jump off the tugboat into the water seconds before it is completely consumed by the river.

The woman filming can be heard saying “Dear God in heaven” as she captures the scene unfold in footage that has since been viewed tens of thousands of times.

In another video of the scene captured seemingly from an aircraft, the woman filming can be heard in distress, repeatedly shouting “My God” and asking: “Have many people died?”

Her video shows the immensity of the Rio Negro (Black River), which is the world’s seventh-largest river by discharge.

According to local media, the tugboat’s crew was rescued alive by the crew of a cargo ship that passed by moments later.

A doctor on board gave the men a checkup and they were found to be in good health. However, they were instructed to see a doctor once ashore to be sure.

According to local media, the Brazilian Navy will investigate the cause of the sinking and will determine if anyone is to be held responsible.

No further details have been given yet of the cause.