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The FBI director has said the row over access to a dead murderer's iPhone was the "hardest question" he had tackled in his job.

James Comey spoke to US politicians as the war of words between the FBI and Apple intensified.

The FBI has asked Apple to unlock the iPhone of Syed Rizwan Farook who killed 14 people in December 2015.

Apple has refused, saying the protection of personal data was "incredibly important".

"This is the hardest question I have seen in government and it's going to require negotiation and conversation," Mr Comey told the House Intelligence Committee.

"I love encryption, I love privacy, and when I hear corporations saying we're going to take you to a world where no one can look at your stuff, part of me thinks that's great," he said.

However, he added, law enforcement saved lives, rescued children and prevented terror attacks using search warrants that gave it access to information on mobile phones.

"We are going to move to a world where that is not possible anymore," he said. "The world will not end but it will be a different world than where we are today and where we were in 2014."

People needed to understand "the costs associated with moving to a world of universal strong encryption", he added.

Mr Comey said it should be left to Congress and others to decide about what counted as reasonable law enforcement access to encrypted data.

He also said if Apple did unlock the phone it was "unlikely to be a trailblazer" for many future cases. The speed with which the technology in smartphones changes meant any defects Apple exploited to get at data would not be present in newer gadgets. Despite this, he added that it would be "instructive" for other courts.

At another hearing Microsoft President Brad Smith said it would be filing an amicus brief to support Apple's position.

Mr Smith told a House Judiciary Committee that Microsoft "wholeheartedly" supported Apple on the issue of resisting calls to unlock the phone.

Earlier this week Microsoft founder Bill Gates said the row should prompt a debate about the access that law enforcement gets to data in terror investigations.

The row between Apple and the FBI over the phone of Farook blew up last week when the bureau asked the electronics firm for help to unlock the smartphone. Farook and his wife Tashfeen Malik killed 14 people and wounded 22 others during the attack in San Bernadino.

So far, Apple has refused to unlock the phone. In an interview aired yesterday with US TV network ABC, Apple boss Tim Cook said the FBI was asking it to make "the software equivalent of cancer".

He added: "Some things are hard and some things are right. And some things are both. This is one of those things."

Apple has until 26 February to formally respond to the FBI order.