Two young men and two women were found dead in a car on Thursday in an apparent Internet-based group suicide, South Korean police said.

Police said the four, including a 25-year-old woman identified only as Park, left suicide notes in their bags in the car parked next to a river in Seongju, 210 kilometres (126 miles) southeast of Seoul.

They apparently inhaled toxic fumes after burning coal briquettes inside the vehicle, which had its windows sealed with plastic tape, police said.

Park's mother in Seoul told investigators that her daughter was reported missing on May 30 after joining a suicide-related website, Yonhap news agency said.

Suicide is the leading cause of death among young people in South Korea, with heavy stress from its highly competitive educational environment and job market taking a lethal toll.

Copycat suicides have increased since an actor killed himself in 2008 by burning coal briquettes in his car.

In 2009 police arrested three website operators in a probe into a series of Internet-based group suicides that left at least 20 people dead.

Portals have since banned websites promoting suicide, and police have expanded a cyber-monitoring team to track down operators of suicide-related websites and blogs, which are illegal in South Korea and punishable by up to 10 years in jail.