ISIS has terror cells operating in England and other European countries that are planning Paris and Brussels-style attacks, the US intelligence chief has warned.

Director of national intelligence James Clapper told journalists in Washington DC that Islamic State militant groups are active in England, Germany and Italy.

"That is a concern of obviously ours and our European allies," he said in response to a question. "We continue to see evidence of plotting on the part of Isil [ISIS] in the countries you name."

Clapper warned that "fanatics" have "taken advantage to some extent of the migrant crisis in Europe, something which the nations have a growing awareness of".

"We've learned that they are... very OPSEC conscious – meaning operational security conscious – they're very mindful of that," he added. "We don't have the total picture all the time – if we did, then the plots would easily be thwarted."

Clapper also said there is a "fundamental conflict" between the desire to retain freedom of movement in Europe while also protecting national security interests.

"The obstacles in Europe have somewhat to do with some of the fundamental conflict between, on the one hand, European Union incentives and drives to promote openness and free movement of people and goods, privacy, which is in some ways in conflict with the responsibility each country has as a nation state to protect the security of its borders and its people," he said. "Those are sort of countervailing processes."

Keith Vaz, chairman of the Commons home affair select committee, told The Telegraph that Clapper's comments reveal "a worrying view which needs to taken very seriously as it comes from a key ally which works with the UK closely on these matters.

"There are worrying concerns about free movement across the EU. Once someone passes through the external border of the EU there are simply no checks before they get to the UK. Even there they are not challenged robustly enough as they are EU citizens. These gaps need to be addressed."

Clapper's remarks follow warnings from the EU and Nato that there is a "justified concern" ISIS militants are plotting to carry out massacres using biological and chemical weapons in Britain and Europe.