As the five-year anniversary of the terrorist attacks of September 11, 2001, approaches, pressure is mounting on local officials to finalize the conceptual agreement with developer Larry Silverstein over the rebuilding of the former World Trade Center site.

The city and state got a boost yesterday when it was announced that two insurance companies, Zurich American Insurance Company and the Employers Insurance of Wausau, gave their assurances that the preliminary agreement that transferred the development of the Freedom Tower to the Port Authority would not interfere in their payment of insurance claims. Pending a settlement in a federal suit, the two companies could owe more than $100 million toward the rebuilding of ground zero.

Still, five insurance carriers that could owe up to $1.4 billion have withheld similar assurances, threatening the conclusion of the development agreement.

A spokesman for Zurich, Keith Owens, said in a statement yesterday: “We recently determined the conceptual framework would not present an impediment to the ultimate settlement of Silverstein’s WTC property claims.”

In April, Mr. Silverstein signed a conceptual framework with the Port Authority, the state, and the city, ending months of bitter negotiations among the parties over the development rights, timetable, and financing of the planned redevelopment. Under the agreement, Mr. Silverstein, who controls a 99-year lease on the site, would transfer the development rights of the Freedom Tower and another tower to the Port Authority, which owns the former World Trade Center site.

While the agreement allowed construction to proceed on the Freedom Tower, Mr. Silverstein and the Port Authority warned that it contained several contingencies that would be difficult to satisfy before a late September deadline, set by the Port Authority, to finalize the contract.

Chief among the developer’s concerns was the transfer of more than $1.5 billion in insurance proceeds to the Port Authority. Seven out of 20 insurance carriers appeared to indicate that they would use the new development arrangement as a pretext to withhold payment.

Public officials, including Governor Pataki and Mayor Bloomberg, sent a flurry of letters aiming to pressure the insurance companies to honor the conceptual agreement. The Port Authority and Mr. Silverstein sued the insurance companies in state court to force them to honor the agreement. The insurers tried to move the case to a federal court, where they are still battling Mr. Silverstein over a host of issues, including the size of the final payout.

This week, a federal judge ruled that the state court was the proper jurisdiction for the dispute, another small victory for the Port Authority and the developer, according to sources familiar with the case.

Yesterday, Messrs. Bloomberg and Pataki and the Port Authority issued statements urging the five remaining insurance carriers — Allianz Insurance Company, Industrial Risk Insurers, Travelers Indemnity Company, Royal Indemnity Company, and Gulf Insurance Company — to provide assurances that they also will accept the conceptual agreement.