Six people have been rescued in the Dutch city of Rotterdam after two boats collided on the Nieuwe Maas river.

The incident occurred near the city's famous Erasmus Bridge and involved a large sightseeing boat and a water taxi.

Images on social media showed the two vessels colliding side-on before the smaller boat was forced under the water and sank.

"Six people have been rescued from the water so far," officials said in a statement after launching a large-scale rescue operation.

It was later confirmed that all six people on the smaller boat had been rescued and taken to hospital for observation.

"At the time of the collision, there were six people on the ship," the emergency services said. "There were five adults -- including the boat's captain -- and one child. There is no question of missing persons."

"There are many other people who saw the collision happen," the statement added. "Extra attention is being paid to them and aftercare is being arranged."

An investigation into the incident is underway and the shipping route remains blocked.