Some 600 British nationals have been stopped from going to Syria to try to join the so-called Islamic State and other jihadist groups, according to British Foreign Secretary Philip Hammond.
Meanwhile some 800 have made it through since 2012, with half of them still thought to be in 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," Mr Hammond said during a visit to southern Turkey.
The foreign secretary said the number of British nationals 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.
He said greater coordination between the two countries had also played a part.
Mr Hammond said besides foreign airstrikes, the interception of jihadists aiming to link up with IS was placing extra strain on the group in its Raqqa headquarters.
"There is evidence (IS) is finding it difficult to recruit to the brigades in Raqqa 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 IS targets in Syria after parliament backed the move in December.
Britain was already involved in attacking IS targets in Iraq.
Syrian govt trying to cut off access to Aleppo
Meanwhile, the Syrian government is reportedly launching a major assault in the northern province of Aleppo, seeking in part to cut off rebel access to the city of the same name.
A security source said the operation aims to prevent rebels from receiving supplies and reinforcements from the suburbs.
One the country's commercial hub, Aleppo is now divided between government control in the west and rebel control in the east.
The province itself is also split, with IS and the regime of Bashar al-Assad present in the east, while rebel groups and the al-Nusra Front, a Syrian al-Qaeda affiliate, occupy parts of the east.
Backed by Russian air strikes, government forces are slowly advancing south of the city.
One commander with the pro-government forces said they are currently fighting on seven fronts across the province and the operation was expected to expand further.
Heavy clashes have been reported between government forces and IS fighters in the north of the province, according to a monitoring group.
The Syrian Observatory for Human Rights said at least 16 IS militants were killed in a failed attack on a regime position.
