South Korea's largest cryptocurrency exchanges were raided by police and tax agencies this week for alleged tax evasion, people familiar with the investigation said on Thursday.

"A few officials from the National Tax Service raided our office this week," an official at Coinone, a major cryptocurrency exchange in South Korea, told Reuters.

"Local police also have been investigating our company since last year, they think what we do is gambling," said the official, who spoke on condition of anonymity. He said Coinone was cooperating with the investigation.