Teva disclosed a 1.35 percent stake in Mylan last week.
"We consider Teva's stakebuilding as a further indication of its intention to meddle with our business, strategy and mission while remaining unclear as to its actual intentions," Mylan said in a letter addressed to Teva's chief executive, Erez Vigodman.(1.usa.gov/1Q0SxgC)
Mylan did not clarify which anti-trust rule Teva violated.
The two companies were not immediately available for comment.
Mylan said on Monday there was still no clarity whether Teva would make a formal offer, almost six weeks since Teva's offer for about $40 billion.
Reporting by Anjali Rao Koppala in Bengaluru; Editing by Maju Samuel