WASHINGTON — The Obama administration argued on Thursday that “no single corporation” — even one as successful as Apple — should be allowed to flout the rule of law by refusing to help the F.B.I. unlock the iPhone used by one of the San Bernardino, Calif., attackers.

The administration’s sharp tone in a new court filing drew an angry and emotional rebuke from lawyers for Apple, who accused the government of “a cheap shot” and were particularly upset about what they said was an unfair and inaccurate suggestion that the company has a special relationship with China to protect its corporate interests there.

“The tone of the brief reads like an indictment,” Bruce Sewell, Apple’s general counsel, told reporters. “In 30 years of practice, I don’t think I’ve ever seen a legal brief that was more intended to smear the other side with false accusations and innuendo.”

The unusually intense sparring between the two sides signaled an escalation in tension over a case that had already drawn attention worldwide because of the high legal and corporate stakes. The fight has been brewing since mid-February, when Magistrate Judge Sheri Pym of the Federal District Court for the Central District of California ordered Apple to create and deploy an alternative operating system that would help law enforcement agents break into the iPhone in the San Bernardino case.