A Yahoo spokesperson in Singapore declined to comment.
The affected accounts appeared to belong to a voice-over-Internet-protocol, or VOIP, service called Yahoo Voices, which runs on Yahoo's instant messenger. The Voices service is powered by Jajah, a VOIP platform that was bought by Telefonica Europe BV in 2010.
The hackers' website where the original claim was made, d33ds.co, was not available later on Thursday. It was registered in February. Industry website CNET reported the hackers as saying the breach was intended as a "wake-up call and not as a threat" and that Yahoo's security was lax.
The Voices hack is one of several in recent months. The business networking service LinkedIn admitted last month that 6.4 million member passwords had been stolen from its website