One US Federal Bureau of Investigation official who has become so frustrated with Apple's iPhone security, he has described its products as the work of an "evil genius".

The technology giant, which has sold billions of products, has made it increasingly difficult to break into iPhones, claiming it keeps customers safer from hackers. But government officials say these measures are getting in the way of justice by blocking potential evidence from suspected criminals.

"At what point is it just trying to one-up things and at what point is it to thwart law enforcement?" said FBI forensic expert Stephen Flatley during the International Conference on Cyber Security in Manhattan this week.

Describing Apple as "jerks", according a report in Motherboard, Mr Flatley revealed that Apple recently made its iPhones even harder to access. It has recently added a trick that makes password cracking software much slower, making it more difficult for law enforcement to break into phones, he said.

Specialists have access to software that tries out every possible set of characters, inputting thousands in a short period of time. But Apple has now limited the attempts it can make at a password per minute, making it difficult for police to open a phone in the already limited time they have to find evidence.