Andrew Parker is the director general of Britain’s domestic security service MI5. (MI5 Security Service via AP)

— Britain’s top domestic security agency chief Andrew Parker warned that the nation faces a significant threat from Islamist-inspired terrorism that shows no signs of letting up anytime soon.

“We face a very serious level of threat that is complex to combat and unlikely to abate significantly for some time,” said Parker, the director general of MI5, in a rare public speech.

Parker’s comments, made at M15’s London headquarters Thursday night, follow the deadly terrorist attack in Paris on Wednesday that left 12 people dead.

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Parker said that about 600 Britons have traveled to Syria or Iraq and some are joining the Islamic State militant organization. He stressed threats were posed from other extremist groups as well.

“We know, for example, that a group of core al-Qaeda terrorists in Syria is planning mass casualty attacks against the West,” he said.

He said that security services have thwarted three terrorist plots in recent months that would have resulted in casualties and that gathering intelligence about plots driven from Syria is challenging.

“There are no in-country partner agencies we can work with in Syria. Collecting intelligence about a war zone is difficult, gathering evidence even more so,” he said.

He also said that “outside Iraq and Syria, we believe that since October 2013 there have been more than 20 terrorist plots either directed or provoked by extremist groups in Syria” and he listed examples in Canada, Australia, Belgium and France.

In August, Britain raised its threat level from “substantial” to “severe” following the rising threat from Syria, he said. Britain has five threat levels — low, moderate, substantial, severe and critical.

Parker also addressed the ongoing tensions between British authorities and U.S. Internet companies, saying that British agents need greater support from Internet companies if they are to prevent attacks.

“If we are to do our job, MI5 will continue to need to be able to penetrate their communications as we have always done,” Parker said. “That means having the right tools, legal powers and the assistance of companies which hold relevant data. Currently this picture is patchy.”