LONDON - BRITISH police are urgently trying to trace a mystery caller who may be able to identify the killers of two Chinese students, a detective said on Wednesday.

The Mandarin speaker said he knew the names of those responsible for the grisly murders of Xi Zhou and Zhen Xing Yang, both 25.

Detective Superintendent Steve Wade said the mystery caller telephoned officers after a police appeal on national television last month.

'That man spoke in Mandarin and in his call he told Chinese-speaking interpreters he knew the names of the killers,' Dectective Wade said.

'We urgently need to get this man to call again. He left a pay-as-you-go mobile number, but we haven't been able to get through to him again as he hasn't answered our calls'.

'I would ask the man to get in touch again as his information could be crucial.'

Their bodies were found with serious head injuries at their flat in Newcastle, northeast England, on Aug 9. Officers believe Zhen may have been tortured for up to an hour before his death.

Dectective Wade added, 'We still believe the lodger who took up the lease holds key information. We still haven't identified him and need to do so urgently.'

Since the deaths, it has emerged that the couple were involved in an Internet betting scam and trading fake documents such as visas and professional qualifications.

Police took the unusual step of issuing a poster in English and Mandarin as they appealed for more information. -- AFP