Islamist extremists fighting the Assad regime in Syria could carry out terrorist attacks in the UK, William Hague has warned.
Syria has now become the "number one destination" for British jihadists seeking to hijack the Arab uprisings, according to the foreign secretary.
Speaking at the Royal United Services Institute, he said the longer the conflict went on, the greater the danger was that battle-hardened militants would pose a threat in the West.
"This includes a number of individuals connected with the United Kingdom and other European countries," he said.
"They may not pose a threat to us when they first go to Syria, but if they survive some may return ideologically hardened and with experience of weapons and explosives."
A Dutch and British photojournalist who were kidnapped in Syria by Islamist militants said they heard British accents among those who were holding them captive after being freed by members of the Free Syrian Army last September.
Estimating that between 30 to 100 held them captive, Dutch photographer Jeroen Oerlemans said some of the gang that held them had "Birmingham accents".
At least one had a "heavy south London accent" reported the Sunday Telegraph, quoting a source close to the incident.
Oerlemans described the men as "foreign jihadists,” who captured the men "almost immediately" after they crossed the border from Turkey at Bab al-Hawa, on 19 July.
Hague also warned that a prolonged struggle in Syria increased the risk that the regime could resort to chemical or biological weapons, urging Russia and China to drop their opposition over UN negotiations for a transition to a new government.
He set out plans for an ambitious programme to build support for human rights in key allies in the fight against international terrorism.
He said the "justice and human rights partnerships" initiative was intended to enable the UK to share intelligence relating to terrorist activity in countries with suspect human rights records without it leading to the torture or abuse of suspects.
They will include assistance to overseas investigators, enabling them to build cases based on evidence rather than confession and to improve their compliance with the law and human rights.
"In many cases, we are able to obtain credible assurances from our foreign partners such as detainee treatment and legal processes that give us the safeguards we need and the confidence that we can share information in this way.
"Where this is not the case, we face a stark choice.
"We could disengage or we can choose to share our intelligence in a carefully controlled way while developing a more comprehensive approach to human rights adherence.
"This approach brings risk, but I am clear that the risks of the first option, of stepping back are greater still, placing our citizens at greater risk of terrorist attack."
The initiative - which comes after six British nationals were killed last month when Islamist militants overran a BP-run gas plant in Algeria - was immediately criticised by civil rights activists.
Cori Crider, the legal director of Reprieve, said: "We've been here before - from Afghanistan to Libya, the UK has handed over detainees or colluded in renditions, knowing that the result will be that people face torture.
"The government has sought to spare its blushes by obtaining 'assurances', but these have not been worth the paper they were printed on.
"William Hague is trying to find a way to join hands with the torturer while keeping his own hands clean - it just won't work."
The government has previously had to pay out millions of pounds in compensation to suspects such as Binyam Mohamed over claims that the British intelligence and security services colluded in their torture while they were detained in countries like Pakistan and Morocco.
Ministers have also so far failed in their long-running battle to deport the radical cleric Abu Qatada to stand trial in Jordan on terrorist charges, despite assurances from the Jordanian authorities that they would not use evidence obtained by torture.
Mr Hague stressed that every aspect of the work would require ministerial approval, and would be halted immediately if there was "any credible evidence" that UK support was being misused.
"This is a framework of accountability and human rights to ensure that our counter-terrorism work supports justice and the rule of law as well as our security, with the goal of creating the long-term conditions for better observance of human rights in countries that have a poor record and where the threat from terrorism is strong." he said.