A subcontractor at Japan's crippled Fukushima nuclear plant told workers to lie about possible high radiation exposure in an apparent effort to keep its contract, reports said Saturday.

An executive at construction firm Build-Up in December told about 10 of its workers to cover their dosimeters, used to measure cumulative radiation exposure, with lead casings when working in areas with high radiation, the Asahi Shimbun newspaper and other media said.

The action was apparently designed to under-report their exposure to allow the company to continue working at the site of the worst nuclear disaster in a generation, media reports said.

A 9.0-magnitude earthquake and tsunami on March 11, 2011 crippled cooling equipment at the Fukushima Daiichi nuclear plant, triggering meltdowns that spewed radioactivity and forced tens of thousands of residents to flee.

The Asahi urged plant operator Tokyo Electric Power to strictly manage the safety of work crews.

The influential daily also called on the government to conduct a thorough survey of work conditions at the site, which has been off limit to the public, except for occasional visits by journalists guided by TEPCO officials.

Several workers at Build-Up told the Asahi that a senior official from the firm who served as their on-site supervisor said in December he used a lead casing and urged them to do the same.

Without faking the exposure level, the executive told the workers they would quickly reach the legally permissible annual exposure limit of 50 millisieverts, according to the Asahi.

The workers had a recording of their meeting, the newspaper said.

"Unless we hide it with lead, exposure will max out and we cannot work," the executive was heard saying in the recording, the Asahi reported.

Some workers refused to wear it and left the company, the Asahi said.

The workers were hired for about four months through March to insulate pipes at a water treatment facility, Kyodo News said.

The ministry of Health, Labour and Welfare was starting to investigate the matter, newspapers and Jiji Press reported.

Health ministry and Build-Up officials could not be reached for comment.