Seoul - South Korea has boosted to $500 000 the reward for tips about the mysterious billionaire who prosecutors say owns a ferry that sank last month.

The ten-fold increase in the reward comes as officials struggle to find Yoo Byung-eun on allegations of tax evasion and embezzlement.

Prosecutors said on Monday that a failure to spend enough money on safety may be a reason for the April 16 sinking.

The sinking of the Sewol ferry left more than 300 people dead or missing, most of them high school students.

The $500 000 bounty is the same amount given if a South Korean citizen provides information about a North Korea-dispatched spy.

Police are also offering a bounty of $100 000, up from $30 000, for Yoo's eldest son.

He faces embezzlement allegations. - Sapa-AP