The chief finance spokesman of the Democratic Party of Japan, Masaharu Nakagawa, told the BBC he was worried about the future value of the dollar.
Japan has been a major buyer of US government bonds, helping the US finance its Federal budget deficits.
But, he added, it would continue to buy bonds only if they were denominated in yen - the so-called samurai bonds.
"If it's [in] yen, it's going to be all right," Mr Nakagawa said in an interview with the BBC World Service.
"We propose that we would buy [the US bonds], but it's yen, not dollar."
What does it say when your allies ponder making the debt you borrow be denominated in a currency they control because they no longer trust you to control your own currency.
It says something bad... very bad.
