JINDO, South Korea -- A passenger ship carrying some 450 people on board ran aground off South Korea's southwest coast on Wednesday, with a rescue operation underway, the Coast Guard said.

A total of some 450 people, mostly high school students on a school trip, were aboard the 6,825-ton vessel when it sent out a distress signal at around 8:58 am in waters 20 kilometers off the island of Byungpoong, officials said.

The Coast Guard said it dispatched patrol boats and helicopters to the scene for rescue operations.

A total of 120 passengers had been rescued from the sinking ferry as of 9:55 am, officials said.

The ferry set off from the western port of Incheon on Tuesday and was to arrive at the southern resort island of Jeju later on Wednesday, the Coast Guard said.

The cause for the stranding and the condition of those rescued were not immediately known.