Seoul: A Korean Air heiress whose tantrum over nuts delayed a flight nearly four years ago is being investigated by South Korean immigration officials on suspicion she unlawfully hired housekeepers from the Philippines.

Cho Hyun-ah on Thursday bowed and apologised for "causing troubles" before entering a Korea Immigration Service office in Seoul for questioning.

Cho and her sister resigned from their executive positions at Korean Air in April following public criticism over their behaviour and allegations their family mistreated and abused employees.

Investigators are also looking into suspicions that the Cho family, including the company's chairman Cho Yang-ho, evaded taxes and used airline services to smuggle luxury goods.

Cho's sister, Cho Hyun-min, is currently under investigation for allegedly hurling a cup of water during a business meeting.