Employers negotiating a labor contract at U.S. East Coast and Gulf Coast ports on Thursday filed an unfair labor practice complaint against the longshoremen's union, saying its leaders refuse to resume talks ahead of a threatened Tuesday strike.

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

The six-year master contract between USMX and the ILA expires Monday, and the two sides appear to be deadlocked on wage issues.

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

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

In rare cases, the NLRB will go to court and ask for an injunction pending the outcome of a board case, but that can take weeks to play out.

The ILA on Thursday responded, calling the USMX a poor negotiating partner.

Earlier this week, ILA leader and chief negotiator Harold Daggett said he had rebuffed USMX approaches.

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

Sources close to the talks said the ILA asked for a wage increase of 77%, a percentage the union called exaggerated. Industry experts predict that the increase will be higher than the 32% raise won by the West Coast longshoremen's union last year.

Companies that rely on ocean shipping are 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 of products such as bananas, meat, prescription drugs, auto parts, construction materials and apparel.

If that happens, delays and costs could quickly cascade, threatening the U.S. economy in the weeks ahead of the U.S. presidential election, burdening already taxed global ocean shipping networks and over time foisting higher prices on consumers.

Economists at Oxford Economics estimated that the impending strike would reduce U.S. gross domestic product by $4.5 billion to $7.5 billion, or 0.1% annualized, for every week it continues.

A strike has the potential to weigh on the October employment report at a time when the Federal Reserve is highly attuned to signs of weakness in the labor market, they said.

The timing is politically sensitive because Democratic Vice President Kamala Harris is facing former Republican President Donald Trump in the U.S. presidential election on November 5.

A White House official on Thursday reiterated that President Joe Biden does not intend to invoke a federal law known as the Taft-Hartley Act to prevent a strike.

"We encourage all parties to come to the bargaining table and negotiate in good faith," the official said. "Senior officials from the White House, Labor Department and Department of Transportation are in touch with the parties and delivering the message to them directly."