The Federal Bureau of Investigation (FBI) said it was able to prevent the Islamic State from launching terrorist attacks in the US during the Fourth of July celebration.

FBI Director James Comey said Thursday the agency uncovered terror plots that were aimed "to kill people in the United States," according to CNN.

US officials told the network that the supposed targets of the terror plots were "coast to coast" with the use guns, knives and other weapons.

Comey said the FBI is seeing ISIS members telling and inspiring people in the US to launch attacks and "sometimes there is a suggestion from an ISIS fighter to do something specific."

He said the technology today affords communication between people in the US and ISIS terrorists to be encrypted.

In the statement he read before the Senate Select Committee on Intelligence on July 8, Comey said "the widespread use of technology propagates the persistent terrorist message to attack US interests whether in the homeland or abroad."

The FBI said US law enforcements have the legal authority to intercept and access communication and information based on court orders but they lack the technical ability to carry out the order because of a fundamental shift in communication services and technologies.

Comey said ISIS threats in the US "remain among the highest priorities for the FBI and the Intelligence Community as a whole."

He said there are at least 200 Americans who have travelled or tried to travel to Syria to join ISIS. While the number is lower, these people "pose a significant threat to the safety of the United States and US persons."

ISIS, he said, has relentlessly promoted hateful message using the Internet and social media.

"ISIL blends traditional media platforms, glossy photos, in-depth articles, and social media campaigns that can go viral in a matter of seconds. No matter the format, the message of radicalisation spreads faster than we imagined just a few years ago," Comey said.

He said the "inspired youth" are more susceptible to be drawn to the ISIS campaign.

"These individuals are often comfortable with virtual communication platforms, specifically social media networks," he said.

Comey cited a Kansas-based male who was arrested last April after he plotted to attack a US military institution and a local police station. The man was inspired by ISIS propaganda and expressed his support to the terrorist group.

Some conversations between people in the US and ISIS occur on social networking sites while others are through private messaging platforms.

Unfortunately, he said, the changing forms of Internet communications are outpacing laws and technology designed to intercept messages.

"This real and growing gap the FBI refers to as Going Dark is the source of continuing focus for the FBI. It must be urgently addressed as the risks associated with Going Dark are grave both in traditional criminal matters as well as in national security matters," he said.

He said law enforcement's authority does not extend to Internet communication services, but that the FBI is using all lawful techniques to combat threats to the US.