JOHANNESBURG, Aug 20 (Reuters) - A cargo ship carrying 147,650 tonnes of coal has broken apart a few hundred metres off of South Africa's Richards Bay port with large parts of the vessel underwater, maritime officials said on Tuesday.

The 230-metre-long ship, named SMART, ran aground in rough seas on Monday. It was not blocking shipping lanes at the Indian Ocean port or impeding operations at the Richards Bay Coal Terminal, the largest coal shipping point on the continent, they said.

"Tugs tried to assist the vessel but to no avail. The vessel has buckled and broken in two," the east coast port's authority Transnet said in a statement.

The single-hull, 151,279 tonne ship is registered to Alpha Marine Corp and flies a Panamanian flag. After loading at Richards Bay it was intended to deliver its cargo to a port in China, according to Thomson Reuters data.

"Currently, there is no environmental impact. There is no oil spillage at this stage," said Preston Khomo, the port manager.

There was no information immediately available on what type of coal it was carrying.

On Monday, 23 crew members including the captain were airlifted off the vessel after it ran aground. No injuries were reported.

Earlier this month, another cargo ship, the 165 metre-long Kiani Satu carrying rice, went aground off the southern coast.

(Reporting by Wendell Roelf and Peroshni Govender; editing by Jon Herskovitz and Keiron Henderson)

Our Standards: The Thomson Reuters Trust Principles.