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LONDON — The British counterterrorism police arrested six people 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 has helped foment jihadism, particularly among young Muslims opposed to British military actions in Muslim regions.

In fact, Britain has been relatively restrained in its support for the U.S. effort to build a coalition against the Islamic State. The British Parliament has approved the deployment of a handful of Royal Air Force warplanes on combat missions against targets, but only in Iraq and not in Syria.

Fearful of being drawn back into a broader conflict after its withdrawal of combat forces from Iraq in 2009, Britain, like the United States, has also ruled out sending ground forces. But it has sent a dozen specialist soldiers to Iraqi Kurdistan to train Kurdish soldiers in the use of 40 heavy machine guns delivered earlier, along with ammunition, equipment and arms supplies from other countries.

The arrests in Britain on Tuesday 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.