Intelligence officials have launched an intensive effort to identify and track New Yorkers and other Americans fighting alongside jihadist groups overseas, amid increased concerns over the rise of Islamic State militants in Syria and Iraq and future U.S. terror attacks, according to a federal law enforcement source.

Federal agents are speaking regularly with informants in Islamic and Arab communities, including some with Long Island connections, the source said, adding that they are also conducting surveillance and monitoring phone and email communications of people they suspect have made contact with known jihadists.

FBI agents and NYPD investigators are closely monitoring intelligence sources in the New York metropolitan area and elsewhere to identify threats, terrorist chatter, or hints of a plot linked to either today's 13th anniversary of the Sept. 11 attacks, the source said, or the group, also known as ISIS.

There is no specific, credible threat to New York, said FBI Assistant director George Venizelos in a prepared statement released last night.

Other officials said they are aware of no credible threats to other potential U.S. targets. Even so, recent gains overseas by ISIS and Islamic militants linked to al-Qaida and the beheading of two American journalists have raised concerns about another stateside attack.

Fueled by jihadists' increasingly sophisticated online outreach efforts, the terrorism climate in 2014 is far more nuanced than it was 13 years ago, officials said.

As a result, the threat posed by radicals with American passports is among the most serious the U.S. has faced since 9/11, they said.

"ISIS creates a totally new area of threat for us . . . probably even more potentially impactful on us than al-Qaida," NYPD Commissioner William Bratton said Wednesday.

The militant group's high-tech, tactics have motivated others to consider committing acts of domestic terrorism, Bratton said.

"The increasing sophistication of their media inspires lone wolves," Bratton said. "Their presence makes the threat against the city more significant than it was."

The danger is not limited to the U.S., officials said. More than 100 Americans are suspected of having traveled to Syria to fight alongside militants, including ISIS jihadists, these officials have said.

At least seven Americans who authorities suspect intended to join Islamic radicals in Syria, have been arrested in the U.S., the source said, and an eighth American was arrested at Kennedy Airport upon returning from the region.

Charges against the eight range from weapons possession and supporting terrorism to lying on a passport application and violating parole or probation, records show.

One of the suspects, Shannon Conley, 19, of Colorado, pleaded guilty Wednesday to conspiracy to provide material support to a foreign terrorist organization.

Officials said she was arrested at a Denver airport in April before she could fly to Syria, where she intended to help ISIS.

The potential for ISIS-linked radicals with American or European passports to strike in America is "what keeps us up at night," the source said, pointing to the case of French national Mehdi Nemmouch, 29, who fought with ISIS in Syria before fatally shooting four people in May at the Jewish Museum in Brussels.

"Knowing where and when we might see that type of attempt here . . . that's where the focus is," the source said.

Concern about so-called lone wolf domestic terrorists spiked after the Boston Marathon bombing, officials said.

Brothers Dzhokhar and Tamerlan Tsarnaev allegedly learned to make pressure cooker bombs from al-Qaida's Inspire magazine and detonated two of the explosive devices near the finish line of the race on April 15, 2013, killing three and injuring more than 260.

Dzhokhar Tsarnaev has pleaded not guilty and is awaiting trial. His older brother was killed in a standoff with police after the bombing.

Terrorist plots like the one in Boston are hard to detect because they rely less on digital and phone communication, which can be monitored, and more on face-to-face planning between handfuls of people, officials said.

Even when intercepted communications or conversations among Americans include pro-jihadist statements, it can be difficult for federal agents to differentiate between empty rhetoric -- protected by the First Amendment -- and actionable intelligence.

"This is a great country with lots of traditions of protecting mouth-running," FBI director James Comey said in August. "We should continue that. But those who are inclined to cross the line, I've got to focus on them."