In an ominous warning issued jointly by the FBI, the Department of Homeland Security and the National Counterterrorism Center, all US law enforcers have been instructed "to be vigilant and prepared" ahead of the July 4th Independence Day holiday in the US following apparently coordinated and nearly simultaneous terrorist attacks in France, Tunisia and Kuwait.

Homeland Security Secretary Jeh Johnson said Sunday he's not discouraging people from attending Independence Day events but cautioned that they must "remain vigilant" and report any suspicious activity.

He said authorities will implement new security measures, some of which will not be visible to the public.

House Homeland Security Chairman Mike McCaul also advised Americans not to dismiss the government's warning. Speaking to "Fox News Sunday," McCaul revealed that "there is a great deal of chatter, a high volume" on terrorist network communications.

He pointed out that a spokesman for the Islamic State has called for jihad during the current Islamic month of Ramadan, which coincides with the one-year anniversary of the establishment of the ISIS caliphate and the US Independence Day holiday.

Moreover, the "Bloody Friday" attacks in France, Tunisia and Kuwait—all within hours of each other, in which ISIS claimed credit—could be just the preview of a bigger ISIS show of strength, authorities warned.

McCaul said foiled terror plots have increased "exponentially" in the past year because of Internet recruiting.

"I'm extremely concerned about the way the Syrian ISIS recruiters can use the Internet at lightning speed to recruit followers in the United States, with thousands of followers in the United States," he said, "And then activate them to do whatever they want to do, whether it's military installations, law enforcement or, possibly, a Fourth of July event parade."

A gunman mowed down tourists on a beach resort in Tunisia on Friday, killing at least 37 people and wounding 36 others. This was followed by the suicide bombing of a mosque in Kuwait, killing at least 25 people. Also on Friday, a man with suspected ties to French Islamic extremists rammed a car into a chemicals plant in Lyon, France, triggering an explosion that injured two people. The severed head of a local businessman was then left hanging at the factory's entrance.

Retired Army Col. Peter Mansoor, who helped design the US military campaign against al-Qaeda, pointed out that the ISIS has become a global threat because of its ability to "evangelise" followers.

"This will only continue unless something is done to destroy ISIL and reduce its appeal to the extremist fringe in the Islamic community," he commented on Sunday.

Earlier this year, Director of National Intelligence James Clapper told Congress that ISIS was a regional threat that would probably only conduct operations in the Middle East. At that time, Joint Chiefs of Staff Chairman Gen. Martin Dempsey also called ISIS "an internal conflict, internal to Islam."

Counterterrorism experts said the ISIS has evolved since then. Moreover, it has become much easier for the terror group's leaders to organise and launch attacks virtually anywhere in the world by their use of social media.

"We do know that ISIL encourages lone wolf attacks," Air Force Col. Pat Ryder, spokesman for the US Central Command, responsible for the Middle East, told reporters at the Pentagon last week.

The experts also cautioned that ISIS may be moving away from al Qaeda's method of waging large attacks, such as the Sept. 11, 2001 attack on the US, warning that the attacks could come more frequently, but with less warning.

"Terrorists at home and overseas are bringing the battle into our homes through the internet," MCaul said Sunday. "We cannot afford to complacently watch the threats mushroom. It is time for action, and to treat this issue like the priority that it is."