WASHINGTON (Reuters) -Boeing said late on Thursday it had filed an unfair labor practice charge with the National Labor Relations Board against the union representing its striking U.S. West Coast factory workers, accusing the leaders of not bargaining in good faith.

The charge is the latest sign of the growing acrimony and increasing frustration in the labor talks as the strike by about 33,000 union members enters its fifth week and piles financial pressure on the struggling planemaker.

Boeing said on Tuesday it had withdrawn its latest pay offer to the International Association of Machinists and Aerospace Workers after two days of talks with federal mediators, citing the union's refusal to seriously consider its proposals.

In a filing with the NLRB, Boeing also accused the union's leaders of misrepresenting the terms of Boeing's offer to its members and of not bringing negotiators to the table with authority to make a deal.

Boeing said the union had engaged in a "pattern of bad faith bargaining" and its "public narrative is misleading and making it difficult to find a solution for our employees."

The union said on Tuesday that Boeing was "hell-bent" about sticking to its "best and final" proposal of a 30% wage increase over four years that it offered last month after the strike began. The union had declined to put the proposal to its members for a vote and said this week it planned a new survey of members.

Boeing noted the union later acknowledged the planemaker had improved its offer, with IAM 751 president and lead negotiator Jon Holden telling Reuters on Wednesday that the proposed changes were "meager".

The union did not immediately respond to request for comment on Thursday. More than 90% of its members voted down an offer for a 25% pay rise last month and it has been seeking a 40% wage increase and the restoration of a defined-benefit pension it agreed to give up in 2014.

Boeing said it remained committed to reaching a compromise to end the strike, which has halted production of its best-selling 737 MAX as well as its older 767 and 777 wide-body programs.

Last month, IAM 751 filed unfair labor practice charges against Boeing. No result has been announced.

The planemaker said it had reluctantly decided to file its charge to "respond to the IAM's legal filings, and to ensure the NLRB other interested parties have an accurate picture of the events of the past few weeks."

U.S. Transportation Secretary Pete Buttigieg said earlier on Thursday it was increasingly important to see a resolution to the strike.

"The solution is going to be one that supports workers, that's compatible with the business succeeding," Buttigieg told reporters at the department's headquarters. "We think both those things are absolutely compatible, and there's a deal to be had."

Asked when the labor stoppage impact would cause concerns about the broader airplane supply chain, Buttigieg did not specify a date but said with "each passing day it becomes more important... for them to come to terms."

A letter sent on Wednesday from 30 House Democrats to Boeing CEO Kelly Ortberg and the union representing the striking workers urged the two sides to bargain in good faith to reach a fair contract in a "timely manner."

To conserve cash, Boeing put thousands of white-collar staff on rolling furloughs and said it would freeze most parts orders except for the 787, made in South Carolina. It also faces a risk of losing its investment grade credit rating.