As of this week, the Port Authority of New York and New Jersey, which manages New York’s JFK airport, will have to find an additional $300,000 to cover an unexpected jump in interest costs.

Municipalities across the United States are experiencing similar shocks, as a corner of the debt markets widely used by such entities becomes the latest casualty of the credit squeeze and the collapse in confidence in the ability of bond insurers to maintain their triple-A credit ratings.