A high-ranking official in charge of handling public complaints is under investigation for "serious violations of laws and discipline", adding to the expanding number of officials scrutinized in the government's recent crackdown on corruption.

Xu Jie, 58, deputy director of the State Bureau for Letters and Calls, is the subject of a probe, the Central Commission for Discipline Inspection of the Communist Party of China said on its website on Thursday.

Xu, who has spent most of his career in the complaint-handling field, was appointed the bureau's deputy head in 2005.