Qari Asim, imam at the Makkah Mosque in Leeds, said the recent tragedy in Tunisia showed that IS were inspiring and radicalising people across the globe.
He described the attack last week as “absolutely despicable” and said he felt “sick to his stomach” when he heard the news.
Mr Asim said IS have emerged as a threat after a period of calm following the July 7 bombings in London - carried out by four bombers, including three from Leeds.
He said: “Since the terrible atrocities committed on 7/7, things have changed, generally, nationally, globally, domestically.
“There’s an even bigger threat now with Isis because al Qaida at that time did not have a state, al Qaida did not have money and power behind them.
“Isis are a completely different phenomena. They have power, they have a state, albeit a lawless state, and they also have access to oil and money. And, as a result, they are portraying a very romantic image of their state.
“So I think the threat is even greater.”
Mr Asim said IS are targeting young, impressionable people.
“What’s happened recently in Tunisia really goes to show, they are attacking people, they are inspiring people across the globe, they are inspiring young people who might be going through mid-life crisis, who might be going through identity crisis, or have been affected due to socio-economic factors, psychological factors or have just really been frustrated with foreign policies of various governments,” he said.
“As a result, I would say the threat has gone bigger due to the factors that are resulting in people being radicalised.
“We are living in a much more unstable world than we did 10 years ago.”
Mr Asim offered the condolences of the Muslim community to the victims of the Tunisia attack.
He said the actions of 23-year-old student Seifeddine Rezgui in the month of Ramadan showed that IS had nothing to do with Islam.
“I felt sick to my stomach when I heard the terrible news of the killings that took place in Tunisia,” he said.
“What happened in Tunisia is absolutely despicable and, if nothing else, this really goes to show that Isil have nothing to do with Islam.
“The perpetrator was inspired by Isil to commit terror in the holy month of Ramadan, which is about showing your devotion to God.
“Through his actions, the perpetrator of the Tunisia attack has really shown that Isil have no regard for faith, they have no regard for God, they have no regard for humanity and human values.”
Mr Asim continued: “The month of Ramadan is about empathising with other human beings and what the perpetrator did in Tunisia was completely the opposite.
“By taking other people’s lives they showed they are an affront of human dignity and an insult to God.”