Swiss Re has gone to court seeking to limit its payment obligations stemming from the Sept. 11 terrorist attacks on the World Trade Center by claiming that there was a single occurrence.

Swiss Re filed a declaratory judgment action yesterday in the U.S. District Court for the Southern District of New York in an effort to limit its payments to the lessees of the demolished WTC.

In an action which was widely expected, the insurer has asked the court to confirm that the Sept. 11 collapse of the twin towers constituted one insurable loss. If this argument succeeds, Swiss Re would be obligated to pay only $3.5 billion, not the $7 billion sought by real estate executive Larry A. Silverstein.

Mr. Silverstein, whose companies hold a 99-year lease on the WTC property, has claimed that each crash by airplanes into the towers was a separate attack.