Islamic State
A police officer crosses a street as he keeps watch on tourists at Times Square in New YorkReuters

Radical Islamic terrorists for Isis (now known as the Islamic State) are now a "very real" threat to New York City due to their mastery of social media, according to the city's police commissioner.

New York City's Police Commissioner Bill Bratton said that the terror group were now harnessing their social media skills to recruit new members who had the potential to attack America from within.

"This is the evolving world of terrorism. We are staying ahead of it," the city's top policeman said.

"We are focusing on it, and I believe that we are as prepared as any entity could be to deal with the threats. But the reality is that we are living in a new era of potential terrorism."

The terror group are now calling for "lone wolf" attacks on the iconic Times Square in online chat rooms affiliated with radical Islamism with instructions on "how to prepare a bomb in the kitchen".

The call to arms also came with a list of tourist destinations that would create "pictures of horror" in addition to the Big Apple.

"Times Square is considered one of the most important and famous tourist attractions in the city," a member of the forum wrote.

"Wear gloves doing the preparation of the work to avoid leaving fingerprints," the post advises.

In further instructions, it said: "Place the [bomb] in a place crowded."

Other targets mentioned in the list were the US State of Texas as well as US and European train tunnels and locations with a high-concentration of military targets.

Bratton now fears that the American members of the 50,000-strong Islamic State, believed to number 100 US citizens, could return from the Middle East to carry out attacks on US soil.

"We are quite concerned with the capabilities of Isis, much more so than Al Qaeda was ever able to project," he said.

"That is the terrible new reality," he said. "There are thousands of potential fighters over there who will exit that conflict. The potential for them to come back and do terrorist acts here is very real and has expanded significantly the terrorist threat."

He added that there are no credible terror threats against the city "based on our latest intelligence" ahead of the United Nations General Assembly next week, where thousands of global representatives are set to attend.