Updated

The head of MI6, Britain's international spy agency, is warning that Britain cannot be safe until the war in Syria is brought to an end.

Alex Younger used his first speech since taking the job to say that the British intelligence and security services have disrupted a dozen plots in the U.K. in the last three years and are conducting hundreds of investigations. Even so, he says it's not possible "to pull up the drawbridge" in increasingly uncertain times.

Younger says "the scale of the threat is unprecedented."

Young described the Islamic State group as a "murderously efficient" organization that is plotting attacks despite facing military pressure in their strongholds in the Middle East.

He says the spy service must take the fight "to the enemy, penetrating terrorist organizations upstream."