British jihadists fighting the Assad regime in Syria could return to carry out terrorist attacks in the UK, Foreign Secretary William Hague has warned.
Mr Hague said Syria had now become the "number one destination" for Islamist extremists seeking to hijack the popular uprisings across the Arab world.
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 return to 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 prolonged struggle also increased the risk that the regime could resort to chemical or biological weapons, he said, and he urged Russia and China to drop their opposition to moves in the United Nations to achieve a negotiated transition to a new government.
Mr Hague used his speech to 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.
"When we detect a terrorist plot originating in a third country, we want to be in a position to share information to stop that planning, and do it in a way that leads to the arrest, investigation and prosecution of the individuals concerned in accordance with our own legal obligations, and with their human rights respected at every stage," he said.
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."