BEIJING (AP) — A labor group says a strike at the world’s biggest athletic shoe maker is snowballing, with about 30,000 of its Chinese workers protesting over insufficient benefits.

Workers in the southern city of Dongguan want Taiwanese-owned Yu Yuen Industrial to make social security contributions required by Chinese law and meet other demands.

They’ve been striking in increasing numbers in on-and-off stoppages since April 5.

The labor unrest threatens to crimp the contract manufacturer’s output for clients that include Nike, Adidas, Reebok, Asics, New Balance and Timberland.

A company spokesman could not be reached for comment.

The strike at the massive, 10-factory complex is the latest in a wave of labor unrest at factories in China, where migrant workers have become increasingly assertive amid a growing shortage of migrant labor.