MEXICO CITY (AP) — The United States said Tuesday it has filed a labor complaint after Mexico refused to act on alleged union-busting by a Mexican company.

The U.S. Trade Representatives Office said it has filed a request for a dispute settlement panel under the U.S.-Mexico-Canada free trade agreement, known as the USMCA.

The trade pact established rapid-resolution mechanisms to guarantee unions the right to organize in Mexico. For decades, wages in Mexico have been held very low because unions were not allowed to organize freely.

Over the last two years, in the face of about 22 requests by the United States, Mexico has usually agreed to press companies to comply. But in January, Mexico refused to act in the case of a call center that allegedly threatened or fired union organizers.

It is only the second time the United States has filed such a complaint. The first complaint was filed in August. Interestingly, it is only in the last six months that Mexico has started refusing the requests.