A Japanese government official who committed suicide this month left a note saying he thought he would be forced to take sole responsibility for forgery related to a government land sale, public broadcaster NHK said on Thursday.

Prime Minister Shinzo Abe's office was aware in early March of potential problems with documents about a controversial land sale to a school operator with ties to Abe's wife, his top spokesman has said, amid suspicions of a cover-up.

Abe and Finance Minister Taro Aso have faced fire since the finance ministry's admission this week that it had altered records involved in the discounted sale of the land in western Japan.

The affair has sparked calls for Aso to quit and could dash Abe's chances of securing another three-year term from September, when the ruling party will hold a leadership election.

Abe has denied wrongdoing by himself or his wife Akie, while Aso has denied he instructed changes to the records.

However, the details of the suicide note left by a finance ministry employee in western Japan who was found dead at his home on March 7, could prompt calls from opposition lawmakers for the resignations of both Abe and Aso.

"Some parts of the documents were too detailed, and my boss forced me to rewrite it," NHK quoted the note as saying. "If this continues, I will be forced to bear all the responsibility by myself."

A police spokesman declined to comment.

Yasunori Kagoike, the purchaser of the land, and his wife were arrested in July on suspicion of illegally receiving subsidies.