Labor authorities conducted on-site inspections at Japan's top advertising agency Dentsu Inc. 10 times during the past decade through December 2015 prior to the alleged overwork-related suicide of one of its young female employees, sources said Thursday.

During investigations into allegations such as illegally long working hours, the company received five warnings for excessive overtime hours, the sources said, hinting the firm could have dragged its feet in improving conditions for staff.

Dentsu has declined comment to Kyodo News about the matter.