Only one thing allegedly stands in the way of the timely completion of New York's tallest skyscraper: a Quebec steel firm.

In a lawsuit filed on Friday, the government agency that owns the World Trade Center site accused ADF Steel Corp. of refusing to deliver the steel required for the antenna that tops the building.

ADF is holding the materials "hostage" until "it receives approximately $6-million [U.S.] allegedly owed under another contract for another project, as ransom," the complaint alleges.

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The contractual dispute over payments and delivery times is complicated by the iconic nature of the project.

Previously dubbed the "freedom tower," the building known 1 World Trade Center is a shining column of glass and steel and the chief symbol of the decade-long effort to rebuild the area devastated by the Sept. 11, 2001 terrorist attacks.

ADF Steel is a U.S. subsidiary of Canada's ADF Group Inc. Headquartered in Terrebonne, Que., the 56-year-old firm has supplied steel for everything from a stadium in Baltimore to Lester B. Pearson airport in Toronto.

The company believes the allegations that it breached its contractual obligations are "without merit," according to a statement released Monday. It "intends to take all appropriate actions to defend its position."

Stuart Sobel, a lawyer representing ADF, noted in an interview that the parties are trying to negotiate a resolution. He also disputed the complaint's allegation that the company had held up any steel shipments. "The site isn't really ready for the steel," he said. "It's being sent to a marshalling yard."

A spokesman for the Port Authority of New York and New Jersey, which owns and oversees the World Trade Center site, declined to comment.

According to Friday's lawsuit, filed in New York Supreme Court, the material ADF is due to supply is the only steel that remains to be installed at the tower. The base and the mast of the antenna weigh approximately 750 tons and are so large they must be conveyed by boat, the company's website says.

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Which brings us to the approaching Canadian winter: If ADF does not ship the giant parts for the antenna by December, the St. Lawrence river will freeze, the suit asserts, making delivery impossible until spring. The result could be layoffs for 100 iron workers while the completion of the tower is "unnecessarily delayed for the foreseeable future," it said.

The Port Authority has asked the court to order ADF to deliver the steel. The Canadian firm is responsible for supplying "unique and indispensable pieces of steel for a project of pre-eminent national importance," the complaint claims. With the 458-foot antenna, the skyscraper will stand 1,776 feet tall, a nod to the year the U.S. declared its independence from Britain.

The lawsuit is part of a complicated dispute over payments due for projects at the World Trade Center site. Early this year, ADF sued the Port Authority and DCM Erectors Inc., the company responsible for erecting the steel. ADF said it was owed more than $11-million for steel it provided for a different tower on the site as well as its transportation hub.

That means ADF isn't the only Canadian firm involved in the brouhaha. DCM is an arm of Toronto's Davis Construction Management Group. Representatives of the company did not respond to requests for comment.