Britain's new terrorism watchdog says the U.K. is facing a level of terror threat not seen since the IRA bombings of the 1970s.

Max Hill made the comment in his first major interview since taking up the role last Monday.

Speaking to The Sunday Telegraph, he warned ISIS was planning "indiscriminate attacks on innocent civilians" and expressed "enormous concern" at the imminent return of hundreds of British jihadists who have been fighting in Iraq and Syria.

He also warned that British teenagers as young as 14 were being radicalized by extremists online.

"The sad fact is the threat in this country represented by what we now know as Daesh, so-called Islamic State, is high, is continuing and is not going to abate," Hill said.

In the interview, he went on to defend ministers over compensation paid to Jamal al-Harith, a British national and former Guantanamo Bay detainee who went on to carry out a suicide attack in Iraq earlier this month.

He successfully sued the government on the grounds British agents knew or were complicit in his treatment while detained by the Americans.

As Britain's new independent terrorism watchdog, Hill will report annually to parliament on the state of British terror legislation.

He'll also conduct his own reviews and has pledge to stand up to Prime Minister Theresa May if he believes her administration's policies infringe personal freedoms.