A Japanese scientist warns visual data suggest that the undersea volcano off Tonga is still highly active almost one week after a massive eruption on Saturday.

Professor Nogami Kenji of the Tokyo Institute of Technology analyzed images captured by a satellite belonging to the US space agency, NASA.

The images taken on Thursday show that a swath of 300 kilometers by 200 kilometers of seawater around the volcano has turned yellowish green.

Nogami says the phenomenon is a result of hydrothermal water mixing with seawater. He says it is the most easily noticeable clue to activities of underwater volcanoes, which are harder to observe than volcanoes on the land.

Nogami also says he has never seen such a vast area of water change color. He says it indicates the volcano continues spewing massive volumes of hydrothermal water.

The professor warns that the volcano may erupt again, although on a smaller scale, and volcanic ash and pumice stones could disrupt air and marine transport.

He says activities of the volcano should be monitored carefully to ensure the safety of people living in the area, as well as aid workers.