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WASHINGTON—More than 90 U.S. senators signed a letter to President Barack Obama pressing him to sanction Iran's central bank, with some threatening legislation to force the move, an outcome that would represent a stark escalation in tensions between the two countries.

Such a measure, if effectively implemented, could potentially freeze Iran out of the global financial system and make it nearly impossible for Tehran to clear billions of dollars in oil sales every month, said current and former U.S. officials.

Many American officials view the blacklisting of Bank Markazi as the "nuclear option" in Washington's financial war against Tehran. Some Iranian leaders have said they would view such a move by the Obama administration as an act of war.

The letter was co-sponsored by Sens. Mark Kirk (R., Ill.) and Charles Schumer (D., N.Y.) in a sign of the bipartisan support for tougher financial measures against Iran. The U.S. fears Iran is developing nuclear weapons, a charge Tehran denies.