"This is the first time the IS has used this medium to inspire a 'lone wolf' type attack," New York City Police Commissioner William Bratton told reporters Wednesday.
"We are quite concerned, as you would expect, with the capabilities of IS much more than Al Qaeda ever was able to project their ability to use social media to try and spread their recruitment efforts and try to inspire," he said.
Bratton noted that New York City was only a plane trip away for IS militants in Syria and Iraq with Western passports.
One of the messages that alarmed authorities was an internet post entitled: "To the Lone Wolves in the United States: How to Make a Bomb From the Kitchen of Your Mom, to Create Scenes of Horror in Tourist Spots and Other Targets."
"We take this all very, very seriously. It is part of a continuum, whether we like it or not in this city, it is part of a continuum we've been living with. There is no specific credible threat and that is an important fact," New York Mayor Bill de Blasio said in an interview with the publication Politico.
The Democratic politician argued that for the city's security, it was necessary that the US kept President Barack Obama's promise of not sending combat troops to Iraq and Syria to fight against the IS.
"It would be absolutely counterproductive for New York City to have American troops involved in this conflict," he said.
Faced with the IS threat, which explicitly mentioned a potential attack on the iconic Times Square in New York City, the municipal police tightened security Wednesday, but said they had no evidence of an imminent attack.
Las Vegas is the other target city that appeared in the jihadis' internet forums, where they actively attempt to recruit new members which has led local police to increase surveillance in the gambling mecca.
The recruitment of Western citizens to fight alongside the IS is of great concern to the US and European governments as those with Western passports could easily return to their homelands to carry out attacks.
Earlier this week, federal officials arrested a US citizen of Yemeni origin on charges of recruiting for the IS and planning attacks on US military personnel and Shia Muslims in New York state.