CHICAGO, Oct. 24 (UPI) -- The United Auto Workers union Wednesday said more than 4,000 workers are on strike against Navistar, the Chicago-area maker of trucks and engines.

The strike, called Tuesday at Navistar plants in six states, is in response to what the UAW called "the company's unfair labor practices," the Chicago Tribune reported Wednesday.

The old contract between the auto workers expired Oct. 1. While Navistar moved medium-truck production from its plant in Ohio to non-union facilities in Texas and New Mexico, production at other unionized plants basically has continued since the contract expired.

In an Monday telephone update for employees, a Navistar negotiating team member said talks would involve "very active bargaining" designed to bring a "swift resolution" to the two sides' differences.

In a communication Tuesday, UAW President Ron Gettelfinger said Navistar's International Truck and Engine subsidiary "has shredded our agreement, shipped our work out of the country, and trampled our nation's labor laws."

The union said it filed federal unfair labor practices claims against the International Truck and Engine group.

UAW negotiators said they are "prepared to return to the bargaining table at any time."