LONDON (REUTERS) - British police arrested a 25-year-old woman on Wednesday on suspicion of terrorism offences related to the ongoing civil war in Syria, the latest arrest in recent weeks amid warnings of a growing threat from Islamist militants.

Officers from London's Counter Terrorism Command said they detained the woman in Bedfordshire, north of London, on suspicion of preparing terrorism acts, and she had been taken to a police station in the capital for questioning.

Two addresses in Bedfordshire were being searched, they added.

Last week, four men appeared in court charged with swearing allegiance to Islamic State (IS) militants, who have seized large swathes of Syria and Iraq, and preparing to launch an attack on policemen or soldiers on the streets of London.

On Tuesday, London police chief Bernard Hogan-Howe said at least five Britons were travelling to Iraq and Syria every week to fight for IS, while the authorities estimate that some 500 Britons have already travelled to the region to join the fighting.

Mark Rowley, Britain's national policing spokesman for counter-terrorism, said last week police had made 218 arrests so far this year and that detectives were carrying out security investigations at an "exceptionally high" pace not seen in years.