(Reuters) -Employers negotiating a labor contract at U.S. East and Gulf Coast ports on Thursday filed an unfair labor practice complaint against the union, saying those leaders refuse to resume talks ahead of the threatened Oct. 1 strike.

The United States Maritime Alliance (USMX) said it filed the complaint with the National Labor Relations Board, due to the repeated refusal of International Longshoremen's Association to return to the bargaining table.

It is uncommon, but not unheard of, for employers to make such complaints NLRB - an independent agency of the federal government that enforces U.S. labor law, particularly with regard to collective bargaining and unfair labor practices.

The six-year contract between USMX and the ILA expires on Sept. 30.

As that deadline approaches, companies that rely on ocean shipping increasingly worried that the ILA's 45,000 members will strike and close 36 ports that handle more than half of U.S. ocean trade, including products like bananas, meat, auto parts, construction materials and apparel.

Delays and costs could quickly cascade, threatening the U.S. economy in the weeks ahead of the U.S. presidential election and burdening already taxed global ocean shipping networks.

A JPMorgan analysis projected a port strike could cost the U.S. economy $5 billion daily.

The employer group said it requested immediate injunctive relief - requiring the union to resume bargaining - so that a deal could be negotiated.

The ILA did not immediately respond to a Reuters request for comment.

In a statement earlier this week, ILA International President and chief Negotiator Harold Daggett said he had rebuffed several USMX offers.

"They call me several times each week trying to get the ILA to accept a low-ball wage package," Daggett said.

(Reporting by Abhinav Parmar in Bengaluru, Lisa Baertlein in Los Angeles and Daniel Wiessner in Albany; Editing by Anil D'Silva and Chizu Nomiyama)