LONGVIEW, Washington (From The Longview Daily News) -- KapStone millworkers have officially gone on strike, union officials said early Thursday morning.

The strike began at 3 a.m. Thursday after the union gave the company a three-hour notice at midnight. Association of Western Pulp and Paper Workers Local 153 officials said the strike is open-ended.

The union is emphasizing that this is an unfair labor practice strike rather than an economic strike. The difference between the two types of strike could determine whether KapStone can hire permanent replacement workers. The National Labor Relations Board may end up deciding that matter, too.

"Kapstone's continued illegal activity is unacceptable; the company cannot continue to break the law," said Kurt Gallow, Local 153 president, in a prepared statement. "The union membership is striking over the company's unlawful conduct."

KapStone officials had no comment Thursday morning.

The union has filed several unfair labor practices charges with the NLRB over failure to furnish information and other complaints. A hearing for one of those charges is scheduled for December.

"The company has shown a callous disregard for the law by failing to provide to the Union information which is critical to the bargaining process," said AWPPW bargaining spokesperson John Minor in a statement.

KapStone Paper and Packaging Corp. officials have previously said they would keep the mill running in the event on the strike.

The union said in the press release that it would "ensure an orderly shutdown and promptly exit the plant site."

Earlier this month, KapStone declared an impasse and imposed its latest contract offer. That offer had been rejected by about 68 percent of voting union members.

The two sides have been at loggerheads over a new contract since spring 2014.

It's been 37 years since Local 153 last went on strike at the Longview mill.

The strike was first authorized by vote in December.