The British counterterrorism police arrested six people on Tuesday in raids across southern England following other detentions in London and the English Midlands.
The detention of three men and three women in Portsmouth and Farnborough, both in Hampshire, and in Greenwich, in southeast London, reflected heightened concerns that the rise of the Islamic State, also known as ISIS or ISIL, has helped foment jihadism, particularly among young Muslims.
The arrests came a day after three men were detained in central London, part of a series of detentions since Britain raised its terrorism threat assessment to “severe” — its second-highest level — in August.
A police spokesman said that the latest arrests were “in relation to conflicts overseas” and that they were “not linked to any immediate threat to local communities or anywhere else in the U.K.”
Two men, ages 23 and 26, and two women, ages 23 and 29, were arrested on charges related to suspected terrorism, the police said, while a 57-year-old man and a 48-year-old woman were detained on suspicion of failing to disclose information about terrorism.
The 57-year-old man was also held on suspicion of “engaging in conduct in preparation of terrorist acts and arranging availability of money and property for use in terrorism,” the police said.
The suspects were not identified by name.
The Press Association news agency said a house in Portsmouth that had apparently been raided by the police was the family home of a British Muslim killed in Syria in December while fighting government troops.
Separately, the sister of John Cantlie, a British journalist held by the Islamic State, appealed to his captors to re-establish what she called “communication through a channel started by you.”
“Sadly, like the families of David Haines and Alan Henning before they were killed, our efforts at reopening dialogue continue to be ignored by those holding John,” his sister, Jessica Cantlie, said in an appeal released early on Tuesday, referring to two Britons shown in Islamic State videos depicting their decapitation.