The US Department of Justice fired back at Apple on Thursday in an ongoing battle over unlocking an iPhone, saying that complying with the FBI's request wouldn't be an "undue burden" for the company.

The government, in a 43-page court filing, said Apple "deliberately raised technological barriers that now stand between a lawful warrant and an iPhone containing evidence related to the terrorist mass murder of 14 Americans."

"Apple alone can remove those barriers so that the FBI can search the phone, and it can do so without undue burden," the government said.

The Justice Department noted that the Constitution, the All Writs Act (the 227-year-old law used to compel Apple to assist the FBI), and the three branches of government should be trusted to "strike the balance between each citizen's right to privacy and all citizens' right to safety and justice. The rule of law does not repose that power in a single corporation, no matter how successful it has been in selling its products."

Apple, during a call with reporters on Thursday, disputed the assertions in the Justice Department's filing and accused the government of taking cheap shots.

"In 30 years of practice, I don't think I've ever seen a legal brief more intended to smear the other side with false accusation and innuendo," Bruce Sewell, Apple's top attorney, said. "I can only conclude the DOJ is so desperate at this point it's thrown all decorum to the winds."

The back-and-forth on Thursday is the latest skirmish in the war between the FBI and Apple, which is resisting a February 16 federal court order to unlock an iPhone 5C tied to December's San Bernardino, California, massacre. The FBI wants Apple to create a special version of its mobile software to help access data on an iPhone used by Syed Farook, one of two terrorists who killed 14 people and wounded 20 others. Apple, which said it has already helped the FBI as much as it can, contends the court doesn't have the authority to force it to write a special version of iOS and has turned this into a broader debate over personal privacy, one that has drawn the tech industry to its side.

The Cupertino, California, company says that complying with the FBI's request will create a back door into the iPhone and set a "dangerous precedent" that exposes all its customers to security risks. The government says this is a onetime request (even though there is a list of a dozen other iPhones it wants unlocked) and argues that getting information from the iPhone is a matter of national security.

Apple on February 25 filed a motion asking the courts to vacate the judge's order requiring it to help the FBI access the iPhone. The company said the order violates its constitutional rights.

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"This is not about one isolated iPhone," said the tech giant in the 65-page document. "Rather, this case is about the Department of Justice and the FBI seeking through the courts a dangerous power that Congress and the American people have withheld: the ability to force companies like Apple to undermine the basic security and privacy interests of hundreds of millions of individuals around the globe."

A 'modest' request

The government on Thursday argued that the "court's order is modest" and applies to a single iPhone.

"It allows Apple to decide the least burdensome means of complying," the DOJ said. "As Apple well knows, the order does not compel it to unlock other iPhones or to give the government a universal 'master key' or 'back door.'"

The FBI disputes Apple's claim that creating software to open up the terrorist's iPhone 5C would affect all other iPhones. The special technology "called for in the order could run only on the subject device," FBI electronics engineer Stacey Perino said in a supporting document filed to the court.

Each iPhone requires Apple to have a specially signed code for each software update, one that exists on that specific device. So the unique identifier on this software would invalidate it for use on other iPhones, Perino argued.

The government defended its use of the All Writs Act, saying in passing the act, "Congress gave courts a means of ensuring that their lawful warrants were not thwarted by third parties like Apple."

William C. Snyder, visiting assistant professor of law at Syracuse and a former federal prosecutor, said he personally used the All Writs Act scores, if not hundreds, of times in the 1990s when prosecuting organized crime and narcotics cases. The act may be seen by Apple as outdated, he said, but it's viewed in the legal community as a well-established, useful tool.

To argue that the All Writs Act doesn't apply in the Apple case "is like arguing that the Writ of Habeas Corpus shouldn't apply to persons held in custody because it is hundreds of years old or that the Fourth Amendment [that protects from unreasonable search and seizure] doesn't apply in cyberspace because it is from a different technological era," Snyder said.

The government also accused Apple of being accommodating to similar requests in China, something Sewell and Apple's other attorneys fiercely disputed. The attorneys called the accusations "ridiculous" and said Apple has never built a back door in its products, nor has any government ever asked it to do so. Only now, in the US, is it facing that question, the attorneys said.

The Justice Department, meanwhile, said that Apple, by its own calculations, would have to set aside as few as six of its 100,000 employees for as little as two weeks to assist the FBI in accessing the iPhone 5C. And the government said Apple's concern that its reputation would be harmed and consumers would lose faith in Apple's security isn't reason enough to allow it to avoid the search warrant. The same argument has been used before, by telecommunications companies and others, but it hasn't stood up in court, the government noted.

The agency added that Apple's rhetoric attacking the All Writs Act as "archaic," calling the FBI's investigation "shoddy," and touting itself as "the primary guardian of Americans' privacy" is wrong and "corrosive."

"The government and the community need to know what is on the terrorist's phone, and the government needs Apple's assistance to find out," the Justice Department filing says.

Apple has until March 15 to file another response to the government's request. A hearing to discuss the standoff between Apple and the FBI is set for March 22 in federal court in Riverside, California.

Here's the full filing:

Government's reply to Apple's motion to vacate

Update, 2:15 p.m. PT: Adds more details from the filing, and lawyer comment.

Update, 4:50 p.m. PT: Adds comments from Apple.