Standard & Poor's says some Argentine bonds are in 'selective default' in what could be the start of a wave of downgrades amid a standoff between the South American country and some creditors.

The ratings agency says it lowered its rating on some Argentine bonds because the country did not make a $539 million interest payment due on June 30.

Argentina tried to make the payment but was prevented by a New York judge as part of an effort to force the country to settle a dispute with U.S. creditors.

S&P said Wednesday it could revise the rating if Argentina were to make the interest payments.

The downgrade came as the economy minister led a last-gasp effort to strike a deal with hedge funds demanding payments of about $1.5 billion.