Fire from a vessel is seen at a port in Ulsan, South Korea on Saturday. Yonhap/ Reuters

A fire onboard two oil tankers that injured nine sailors at a South Korean port has been put out, the Yonhap news agency said on Saturday.

An explosion occurred on the Stolt Groenland, a 25,000-tonne, Cayman Island-flagged oil tanker, at the southern port of Ulsan at around 10:50am (0150 GMT), Yonhap reported, citing the Coast Guard. There were 25 sailors aboard, including Russians and Filipinos, Yonhap said.

The fire spread to another unspecified oil tanker docked nearby, it said.

All of the sailors on the Stolt Groenland and another 21 aboard the other ship were rescued, Yonhap said. Nine sailors were wounded including one in critical condition, the report said, without specifying what ship they were on.

The Coast Guard is investigating the cause of the incident and police have blocked traffic in the area in case of a further blast, Yonhap said.

Reuters