British police said on Thursday they had arrested nine men on suspected terrorism offences as part of what they described as an ongoing operation into Islamist-related terrorism.

The men were held on suspicion of belonging to and supporting an organization banned under terrorism laws and encouraging terrorism, police said.

The arrests were not in response to any immediate threat, Scotland yard said in a statement.

Police said the men, aged between 22 and 51, were in custody at stations in central London while 19 properties across the capital and in Stoke-on-Trent in central England were being searched.

Britain last month raised its international threat level to the second-highest level of "severe", meaning an attack is considered highly likely, while Prime Minister David Cameron has said the Islamic State militant group in Syria and Iraq poses the country's greatest ever security risk.