Apple has said the US government's potential ability to unlock a phone used by one of the San Bernardino killers means the company may not have to provide help in a similar case in New York.

The tech giant wrote a letter to a federal court in Brooklyn on Thursday, setting out its position after the government said it may have found a way to crack the shooter's iPhone without Apple's help.

Its lawyers asked to delay by at least 10 days a March 31 deadline to respond further in the New York case until after the government submits a report on the status of the San Bernardino phone on April 5.

"If that same method can be used to unlock the iPhone in this case, it would eliminate the need for Apple's assistance," the letter said.

Similarly, if the government says the method does not work, Apple would want time to test that claim, it added.

"Briefly staying the briefing schedule to allow these matters to crystalize is appropriate in light of the public importance of this issue," the lawyers wrote.

The phone in the New York case belonged to Jun Feng, who has pleaded guilty to conspiring to distribute methamphetamine.

In February, Magistrate Judge James Orenstein ruled that police had overstepped the law when they called on Apple to unlock his phone. Judge Margot Brodie is hearing an appeal.