The head of British intelligence agency MI5 has warned that a group of Al-Qaeda terrorists in Syria is planning "mass casualty attacks" against the West.

Andrew Parker, director general of the agency, said transport networks and iconic landmarks were among Western targets of "complex and ambitious plots" by Syria-based extremists.

Aviation bomb plots and shootings in crowded areas of major cities are thought to be among the plans being developed by the group, according to Mr Parker.

It is understood the organisation in question is the so-called "Khorasan" cell, which is made up of veteran jihadists sent to Syria by Al-Qaeda leader Ayman al-Zawahiri.

Khorasan is embedded within Al-Qaeda's Syrian branch Al-Nusra Front, which is known to include radicalised Britons who have travelled to the war-torn country to fight.

Mr Parker said "we know that a group of core Al-Qaeda terrorists in Syria is planning mass casualty attacks against the West."

He added: "Al-Qaeda continues to provide a focus for extremists to plot terrorist attacks against the West."

Khorasan has been described as a small group made up of around 50 fighters, including expert bomb makers and high-ranking members of Al-Qaeda who moved to Pakistan following the 2001 invasion of Afghanistan.

US officials said they had been sent to Syria not to fight the government of president Bashar Assad but to "develop external attacks, construct and test improvised explosive devices and recruit Westerners to conduct operations".

In September, US military forces carried out air strikes against Khorasan after intelligence reports suggested that the group was in the final stages of plans to launch major attacks against Western targets including the United States.

Mr Parker also added that around 600 extremists have now travelled to Syria - higher than previous estimates of 500, with a significant proportion joining the Islamic State group.