Four men were being questioned today on suspicion of instigating acts of terrorism, police said.
Dorset Police and the UK Border Agency initially arrested five men in their 20s and 30s and a 16-year-old boy on suspicion of illegal entry into the UK in Ashley Road in Parkstone, Poole, on 9 December.
Following an investigation, four of those, including the youth, were later arrested under section 40 (1)(b) of the Terrorism Act 2000 dealing with the commission, preparation or instigation of acts of terrorism.
On Saturday, a district judge at Bournemouth Magistrates Court granted officers permission to extend their detention until Thursday.
The four were then taken to a police station outside Dorset for further questioning.
A Dorset Police spokesman said: "While the investigation is continuing there is currently no identified threat of any risk or harm to the people of Dorset or the United Kingdom."