PARIS — Alarmed by the threat a soaring currency poses to Switzerland’s economy, the country’s central bank sought Tuesday to cap the value of the franc against the euro and said it was prepared to spend an unlimited amount to defend it.

The Swiss currency, long considered a haven in crises, has surged against the euro and the dollar this year as investors have fled turmoil in the markets. That has raised fears among Swiss companies that the country’s exporters will be priced out of business abroad.

“The current massive overvaluation of the Swiss franc poses an acute threat to the Swiss economy and carries the risk of a deflationary development,” the Swiss National Bank said in a statement. It said it was “therefore aiming for a substantial and sustained weakening of the Swiss franc.”

The central bank “will no longer tolerate” an exchange rate below 1.20 francs to the euro, and “will enforce this minimum rate with the utmost determination and is prepared to buy foreign currency in unlimited quantities,” said the statement, which was unusual for its strong wording.