The United States lacks a credible plan to cut its deficit over the medium term, the International Monetary Fund's chief economist Olivier Blanchard told French daily Le Monde in an interview published on Wednesday.

There are reasons to be worried. The United States lacks a credible plan, for the medium term, to reduce its budget deficit, Blanchard was quoted as saying, after rating agency Standard & Poor's signaled on Monday that the United States' triple-A credit rating was under threat.

Blanchard also told Le Monde that euro zone peripheral countries must considerably improve their productivity or trim salaries, or both, to get their economies back on track.

(Reporting by Jean-Baptiste Vey; editing by Patrick Graham)