US agents cannot access a telephone used by the Islamist attackers in the San Bernardino shooting, the head of the FBI said Tuesday, complaining that encryption is hampering investigations.

FBI Director James Comey revealed the problem as he was briefing lawmakers on emerging threats, using it to underline the US government's concerns about the strength of commercially-available encryption.

"It affects our counterterrorism work," he said.

"In San Bernardino, a very important investigation to us, we still have one of those killers' phones that we've not been able to open. It's been over two months now and we're still working on it."

On December 2 last year, US citizen Syed Farook and his Pakistani wife Tashfeen Malik stormed a workplace social event and shot 14 people dead in San Bernardino, California.

Since the attack, the FBI and US intelligence have been trying to work out to what extent the couple were self-radicalized and whether or not they had links to broader jihadist-inspired groups or individuals.

Clapper's anecdote about the suspect's phone feeds into a broader campaign that the US government is waging to persuade Silicon Valley tech giants to give it access to encrypted devices and online files.

Consumers, worried by cyber crime and government snooping, are increasingly drawn to protected products and the industry is keen to serve the market, despite official concerns that encryption empowers criminals.

"Those devices are going to hold the evidence of child pornography, communications that someone made before they were killed, before they went missing," Clapper said, describing phones with a default lock.

"So it is a big problem with law enforcement armed with a search warrant when you find a device that can't be opened even though the judge said there's probable cause to open it."