An iceberg, 25 meters in length and 20 metres in height, has been spotted south-east of St Francis Bay, Eastern Cape, the National Sea Rescue Institute (NSRI) said on Monday.

"This is very unusual and in fact we don't know of anything in recent history that has being reported this close to South African waters," said NSRI spokesman Craig Lambinon.

The iceberg was spotted around 6pm on Monday by those aboard a fishing vessel, the Ntini. It was floating 35 nautical miles south-east of St Francis Bay.

Lambinon said maritime authorities were taking the report "quite seriously" and the maritime radio services were broadcasting a warning to vessels in the area.

The situation would be monitored throughout the night to try to determine the direction and speed of the drift of the iceberg.