Appen says it cannot explain why its share price rose 30 per cent in four hours on Tuesday despite confirming it received a confidential takeover offer valuing the struggling data services group at $154 million.

The buyout proposal, lobbed by Nasdaq-listed Innodata, was only disclosed following queries from the ASX, which stopped the trading of Appen shares on Tuesday after they spiked from 87¢ to $1.11 without the company having made any announcement.