The head of the Federal Bureau of Investigation (FBI) said last week that the Islamic State jihadist group has been recruiting potential homegrown terrorists "24 hours a day" in the United States.

FBI Director James Comey said social media is being used to lure them to go overseas to join extremists "or if you can't come, kill where you are," said Comey said, according to the Associated Press.

"And that is a two-pronged siren song that goes out through social media," Comey added.

He said extremists use Twitter and encrypted communications to recruit terrorists, a method that has become pervasive over the past two years.

Comey said the FBI is conducting investigations in all 50 states relative to the ISIS recruitment.

"So we have investigations in all 50 states to try to identify, among hundreds of people who are consuming this poison, where are they on the stages from consuming to acting, and how do we stop them from acting," he said.

The FBI Ohio region has made two arrests this year of young men who are facing charges of plotting terrorist acts and have pleaded guilty.

He said authorities try to pinpoint those recruits who need mental health care but warned others that they will be punished.

"This is not a game. If we catch you doing this, you're going to get a very long stretch in jail," he said.

Comey added that another FBI concern is the increase in violent crimes in US cities including Cincinnati, adding that the spread of heroin has played a role in some cases.

A congressional Homeland Security Committee report released early this month showed that Minnesota has the highest number of American recruits for ISIS.

It said more than 250 Americans "have tried or succeeded in getting to Syria and Iraq to fight with militant groups."

These people were "individuals who were stopped before traveling, who made it to the conflict zone and are still there, who were killed, and others who have come back."