WASHINGTON, May 7 (Reuters) - U.S. Treasury Secretary Janet Yellen on Sunday warned that a failure by Congress to act on the debt ceiling could trigger a "constitutional crisis," with consequences for financial markets and interest rates. Yellen in an interview on ABC News's "This Week" said debt ceiling negotiations should not take place "with a gun to the head of the American people," and reiterated a warning to lawmakers the government could pay its bills only through early June without increasing the limit, which the government hit in January. Deputy Treasury Secretary Wally Adeyemo also sounded the alarm on Sunday about the risks of a default during an interview on MSNBC's "The Sunday Show." "If we were to default on our debt, it would have a terrible impact on interest rates." Reporting by John Kruzel, editing by Deepa BabingtonOur Standards: The Thomson Reuters Trust Principles.

---