Japan's big four auditing firms have expressed concern about auditing Japan Highway Public Corp.'s financial statements by the end of August, sources said Wednesday.

During talks held from Thursday through Tuesday, the four auditing firms told the government-run firm they will need more time and more specific accounting criteria if they are to audit its financial statements, the sources said.

The highway operator is expected to select an auditing firm to handle the job from among the big four through a tender.

Last week, Chikage Ogi, minister of land, infrastructure and transport, told Japan Highway to have its financial statements audited by the end of August.

Ogi gave the order in response to allegations that the company hid financial statements showing it has a negative net worth of 617.5 billion yen.

The minister said she believes the statements are official and accurate but need to be audited again to make sure.

The authenticity of Japan Highway's financial statements came under scrutiny following a recent allegation by Sachio Katagiri, deputy head of the public corporation's Shikoku branch, in a monthly magazine that Japan Highway President Haruho Fujii had decided against releasing the statements, which were compiled in 2002.

Fujii denied the allegation when he spoke in the Diet earlier this month.

An official balance sheet released June 9 shows Japan Highway's net worth is positive, with its assets exceeding debts by 5.76 trillion yen as of March 31.