Islamic extremists from Western countries who have gone to fight in Syria could carry out terrorist attacks when they return home, British Foreign Secretary William Hague warned on Thursday.

Hague said war-torn Syria had become the "number one destination" for jihadists from around the world.

"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," he said.

"The longer the conflict continues, the greater this danger will become," he added in a wide-ranging speech setting out Britain's plans to tackle terrorism without compromising human rights.

British trainee doctor Shajul Islam and another man, Jubayer Chowdhury, are due to go on trial in Britain in June charged with the kidnapping of two Western journalists who were held by Islamic extremists in Syria.

British photographer John Cantlie has said he and Dutch journalist Jeroen Oerlemans were held for a week last July by some 30 Islamic militants from countries including Britain, Pakistan and Chechnya.

Hague warned that a prolonged war in Syria -- which has already suffered 22 months of unrest, claiming some 70,000 lives -- would also increase the risk of chemical or biological weapons being used.

He called on Russia and China -- who have previously blocked action against Syria at the United Nations Security Council -- to work with other countries to negotiate a new Syrian government formed from the opposition and elements of the regime.

In terms of wider British foreign policy, Hague said Britain would build "justice and human rights partnerships" to help share intelligence with countries with suspect rights records without this leading to suspects being tortured.

"A large part of our effort to counter terrorism is now overseas where terrorists train and plan for attacks against the UK or our interests abroad," he said. "We cannot do this without working with other countries."