A former New York state official has warned that the entire U.S. federal and state prison systems have become a "breeding ground" for Islamic terrorists.

Home-grown terrorists confined in these prisons could radicalise fellow inmates, who could end up as lone wolf terrorists when they are released, experts warned, according to Fox News.

"If we continue to downplay the threat, we do so at our own peril," says Patrick Dunleavy, author of "The Fertile Soil of Jihad: Terrorism's Prison Connection" and retired deputy inspector general of the Criminal Intelligence Unit of the New York Department of Correctional Services.

The number of convictions of domestic terrorists has reportedly risen as the Islamic State (ISIS) began ramping up its recruitment in the U.S. About 71 convicts have been sent to prison for ISIS-related charges including 56 in 2015, according to the George Washington University's Program on Extremism.

The FBI is also investigating 900 cases of ISIS-linked radicalisation.

About 100 inmates serving time on terrorism-related sentences are due for release in the next five years, according to the Congressional Research Service. More terror suspects may be transferred to U.S. prisons from Guantanamo Bay in the coming months.

"We have never been faced with such a large number of terror inmates before," said Republican Rep. Peter King during a recent Homeland Security Committee hearing on countering violent extremism in prison.

The U.S. Bureau of Prisons should monitor and isolate these inmates, King said.

Dunleavy warned that criminals have been radicalised in prisons for years, warning that the situation could get worse.

He cited the case of Chicago gang member Jose Padilla, who converted to radical Islam while in prison in the 1980s and was accused of planning to detonate a "dirty bomb" in the U.S. Padilla is currently serving a 21-year sentence for conspiring to commit acts of terror overseas.

Dunleavy said many other inmates in state and federal prisons have also been radicalised, either by listening to fellow inmates or hearing sermons on contraband devices smuggled into prisons.

Mark Hamm, professor at Indiana State University, said thousands of federal prisoners have converted to Islam and other religions. However, he said most of the converts do not subscribe to violent interpretation of their faith. But he then added that it takes only a few radicalised inmates to create a threat.

"It is not the sheer number of prisoners following extremist interpretations of religious doctrines that poses a threat. Rather, it is the potential for the single individual to become radicalised," he said.

The FBI said radicalised inmates are a concern as they could urge other prisoners to attend radical mosques when they are released; could incite violence in prison; and could pass skills used in terrorism activities.