Some 600 Britons have been stopped from going to Syria to try to join Islamic State and other Islamist militant groups, Foreign Secretary Philip Hammond said in comments reported Saturday.
Meanwhile some 800 have made it through since 2012, with half of them still thought to be inside the war-torn country, he said, in comments reported in The Guardian and The Daily Telegraph newspapers.
"Approximately 800 Brits have been to Syria, of whom half are still there. But on top of that 800, we have stopped another 600," he said, on a visit to southern Turkey.
The foreign secretary said the number of Britons stopped in Turkey had gone up in the past eight months due to Ankara reassessing the scale of the threat posed to Turkey by Islamic State (ISIS).
He said greater coordination between London and Ankara had also played a part.
Hammond said besides foreign airstrikes, the interception of Islamist militants aiming to link up with ISIS was placing extra strain on the group in its Raqa headquarters.
"There is evidence (ISIS) is finding it difficult to recruit to the brigades in Raqa because of the high attrition rate of foreign fighters," he said.
"Not just those targeted in UK drone strikes, but US strikes against prominent targets including foreign fighters.
"Generally they are very stretched now -- their manpower on the ground in relation to the territory they're holding is very thin."
British fighter jets joined the US-led coalition bombing ISIS targets in Syria after parliament backed the move in December.
Britain was already involved in attacking ISIS targets in Iraq.