Homeland Security Secretary Jeh Johnson says his department is encouraging law enforcement "to be vigilant and prepared" ahead of the July 4th holiday in the U.S. following attacks in France, Tunisia and Kuwait.
Johnson says people should attend Independence Day events as planned but "remain vigilant" and report any suspicious activity.
He says U.S. authorities will adjust security measures, including those unseen by the public, as necessary.
On Sunday, House Homeland Security Chairman Mike McCaul said Americans should heed the government's warning.
"It is concerning," McCaul told "Fox News Sunday." There is a great deal of chatter, a high volume," he said of terrorist network communications.
He noted that a spokesman for the Islamic State (ISIS) has called for jihad during the Islamic holy month of Ramadan — currently under way — the one-year anniversary of the establishment of the ISIS caliphate and the American Independence Day holiday.
In addition to that confluence is the "Bloody Friday" attacks in France, Tunisia and Kuwait, all within hours of each other, in which ISIS claimed credit.
The warning to Americans was issued jointly by the FBI, the Department of Homeland Security and the National Counterterrorism Center.
"I think given these confluence of events we're being on the cautious side here to warn the public to remain vigilant," McCaul said, "to enjoy the Fourth of July parades, but remain vigilant during these celebrations."
Foiled terror plots have increased "exponentially" in the past year, McCaul said, 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 killed at least 37 people and wounded 36 in an attack on a beach resort in Tunisia Friday. In Kuwait, a suicide bomber killed at least 25 people, while a man with suspected ties to French Islamic radicals rammed a car into a gas factory in southeastern France, triggering an explosion that injured two people. The severed head of a local businessman was left hanging at the factory's entrance.
While there was no specific or credible threat of attack, one law enforcement official told USA Today that a new intelligence bulletin is alerting local colleagues to the ongoing threats posed by the group that calls itself the Islamic State [also known as ISIS or ISIL] and other homegrown extremists. The official was not authorized to comment publicly.
The bulletins are frequently issued in advance of major U.S. holidays out of an abundance of caution and concern that operatives may exploit the timing to generate greater attention.
The FBI and other agencies have worked to disrupt a number of Islamic State-inspired plots, including a planned assault earlier this month on police officers in Boston. In that case, authorities fatally shot Usaamah Rahim as he allegedly planned to attack police with military-style knives.
