U.S. grocery chain Kroger countersued Albertsons on Tuesday, escalating a legal battle between the companies following the collapse of their proposed $25 billion merger in December.

In court papers filed Tuesday in the Delaware Court of Chancery, Kroger said Albertsons disregarded the companies’ merger agreement and worked secretly with a partner, C&S Wholesalers, to try to force Kroger to divest more stores to C&S.