Tim Cook: The FBI wants us to write the software equivalent of cancer

“What’s at stake here is, can the government compel Apple to write software that we believe would make hundreds of millions of customers vulnerable around the world -- including the U.S. -- and also trample civil liberties,” Cook said.

He made his remarks in a 30-minute interview that aired on ABC News Wednesday evening. The CEO was pressed repeatedly on why Apple shouldn't make an exception for a single iPhone that was used by a terrorist.

“This is not about one phone, this is about the future,” Cook said.

Besides compromising people's security, Cook said complying with the FBI would set Apple on a slippery slope that could lead to more invasive requests in future.

"Think about what else they could ask us to write. Maybe it’s an operating system for surveillance, maybe it’s the ability for law enforcement to turn on the camera," he said.

"I don’t know where this stops, but I do know this is not what should be happening in this country."

The FBI wants Apple to write software that would allow investigators to circumvent protections on the iPhone lock screen and access information stored on a phone used by one of the perpetrators of the December mass shooting in San Bernardino, California.

A magistrate judge last week ordered Apple to comply with the request, but Apple has refused and is due to file an appeal by Friday.

Such decisions shouldn't be made in courtrooms on a case by case basis, Cook said, and urged Congress to pass laws after a public debate.

"At the end of the day, we have to follow the law like everybody else," he said, suggesting Apple will go along with what legislators decide.