The United States on Friday paid $85.6 million to the financially beleaguered United Nations, reducing its debt on the regular budget to $366.3 million.

The payment was made one day after the Soviet Union, which is paying its own arrears, pledged the equivalent of $600 million to U.N. relief and reconstruction efforts in Afghanistan.A U.N. statement said the payment was the second installment of the $188 million that President Reagan promised to pay by the end of the year. The United States earlier paid $15.2 million.