SEOUL (AFP) - South Korean officials on Monday launched a probe into cram schools after a cheating scandal that led to the nationwide cancellation of US college entrance exams earlier this month.

The education office in the capital Seoul started the special investigation into a number of private academies suspected of illegally obtaining questions for the SAT test and offering them to pupils, a spokesman told AFP.

The administrator of the SAT - the most widely used test for applying to US colleges - scrapped the scheduled May 4 exams after discovering questions were already circulating among some test-prep schools.

The College Board also cancelled subject tests in biology scheduled for next month.

"The moral hazard prevalent among some SAT prep schools has reached a serious level," the Seoul education office said in a statement, criticising school operators of "tarnishing the national reputation and harming innocent applicants".

Those discovered to have leaked questions will face closure as well as special tax audits, with the owners banned from opening new schools for a certain period, it added.

Cram schools for global tests - not to mention domestic college exams - are a lucrative industry in education-obsessed South Korea where qualifications from top colleges are crucial for careers and marriage prospects.

In 2010, a teacher at a Seoul cram school was arrested for smuggling SAT questionnaires out of a test held in Thailand and sending them to his pupils scheduled to take the same exam hours afterwards.