At the delivery company’s primary U.S. air hub, over 1,100 recently organized DHL Express employees went on strike on Thursday in protest of unfair labor practices and the ongoing impasse over contract negotiations.

During the crucial holiday shipping season, when package carriers like DHL, FedEx (FDX.N), and United Parcel Service (UPS.N) see a boost in business, the strike poses a risk of shipment delays.

It also coincides with the International Brotherhood of Teamsters stepping up its organizing efforts following the widely reported contract agreement at UPS, the largest delivery company in the world, this summer.

The workers, who load and unload DHL Express airplanes at Cincinnati/Northern Kentucky International Airport (CVG), voted to organize with the Teamsters in April. They have been negotiating their first contract with DHL since July.

“DHL Express was fully prepared for this anticipated tactic and has enacted contingency plans” to ensure that service is not disrupted, the company said in a statement. Those plans include using replacement staff at the hub and moving flights and volume away from CVG to other DHL locations throughout the Americas region.