London (AFP) - The head of Britain's domestic spy agency MI5 warned on Thursday that militant Islamists in Syria were planning "mass casualty attacks" in the West and that intelligence services may be powerless to stop them.

"We know... that a group of core al-Qaeda terrorists in Syria is planning mass casualty attacks against the West," Andrew Parker told journalists in London a day after an Islamist attack in Paris claimed 12 lives.

"Although we and our partners try our utmost, we know that we cannot hope to stop everything," he added.

Parker said that fighters returning to the West from the civil war in Syria were in danger of bringing with them a "twisted ideology" that could lead them to carry out attacks on famous landmarks at home.

Although Islamic State (IS) presented the most obvious threat, fighters aligned with the core of al-Qaeda were also a danger.

"We still face more complex and ambitious plots that follow the now sadly well-established approach of al-Qaeda and its imitators -- attempts to cause large-scale loss of life, often by attacking transport systems or iconic targets," he said.

The MI5 director general earlier said the attack against French satirical magazine Charlie Hebdo was a "terrible reminder" of the threat facing Western nations, revealing that his agency had helped stop three terror plots "in recent months alone".

Parker called for wider powers to identify and monitor suspects, following a backlash against the security services after the extent of their snooping was exposed by Edward Snowden, a former National Security Agency contractor.