A "substantial number" of British citizens are fighting with extremist groups in Syria and could return to launch terror attacks against the UK or Western allies, William Hague warned.
The Foreign Secretary said violent Islamists enjoyed sufficient "uncontested space" in the war-torn country to provide foreign recruits with "extensive training".
"This is particularly concerning as we assess some of the individuals being trained will seek to carry out attacks against Western interests in the region or in Western states now or in the future," he said in a stark assessment of the potential threat.
Mr Hague set out Britain's fears after being challenged by the Commons foreign affairs select committee to justify a decision to push for the further easing or lifting of an EU arms embargo.
Britain and France are spearheading the move - which would allow the arming of rebel groups - but face opposition from partners concerned weapons could fall into the hands of jihadists.
Richard Ottaway, the Tory MP who chairs the committee, told Mr Hague in a letter last month that it was concerned the UK was "out of step" with almost all European partners on the issue.
In his reply, sent at the weekend and published by the committee, Mr Hague said there was no doubt that the two-year battle between rebels and the forces of president Bashar Assad "poses current and future risks to UK national security".
"As the conflict has progressed, violent Islamist groups have been gaining ground in Syria and have attracted a large number of foreign fighters of all nationalities, including a substantial number of UK citizens," he said.
"The overall number of foreign fighters travelling to Syria is greater than for all other areas of jihad combined. There is enough uncontested space in Syria for some violent Islamist groups to provide extensive training."
Mr Hague said there was a "real and concerning kidnap threat to British nationals" and a risk of "spill-over terrorist-style attacks" in Lebanon and Jordan that could affect UK interests.
https://www.gov.uk/government/organisations/foreign-commonwealth-office(Foreign and Commonwealth Office)