LONDON (AFP) - British police are grappling with "exceptionally high" numbers of terrorism investigations and foiling several plots a year, the counter-terrorism chief warned on Friday.

"So far this year we have made 218 arrests and are running exceptionally high numbers of counter-terrorism investigations, the likes of which we have not seen for several years," national policing lead for counter terrorism Mark Rowley said in a statement. "We are disrupting several attack plots a year."

Mr Rowley said that the terrorism plots were "of varied sophistication, from individuals planning to carry out spontaneous yet deadly attacks to more complex conspiracies".

Almost all appeared to be directed from abroad or inspired by overseas terrorism, Mr Rowley said. It comes after the police arrested several people this month in what they said was a "significant" plot.

Britain raised its terror threat level to "severe" in August for the first time in three years.

The government has expressed concern that British-born militants who have travelled to Iraq and Syria could pose a threat if they return.

Over 500 British militants are estimated to be fighting in the two countries.

Mr Rowley said that extremist groups are using social media in increasingly sophisticated ways, and that the police remove over 1,000 pieces of extremist material from the Internet each week.

"The growing problem of young, impressionable, and in some cases vulnerable, individuals being radicalised online is an increasing risk," Mr Rowley said.

Much of the material relates to Iraq and Syria and some relates to beheadings, suicide and torture, Mr Rowley said.

In all, 66 people have been reported missing to the police are feared to have travelled to Syria, according to the anti-terror chief.