Syria's president Bashar al-Assad is also continuing a crackdown against pro-democracy protesters, but Mr Hague denied that hopes for democracy in the Arab world were fading.
"We should remain on the optimistic side of what is happening in the Arab Spring, although there will be many conflicts and difficulties along the way," he said. The Foreign Secretary attacked the behaviour of Assad's regime in Syria as "appalling and unacceptable" and vowed the UK would "do what we can to support democracy in the future".
He is meeting with Syrian opposition leaders in London today but warned against comparisons with Libya, where UK forces helped overthrow Colonel Gaddafi, instead saying the UK would "increase the pressure" on Assad. Mr Hague also called for Colonel Gaddafi's son Saif, captured in Libya at the weekend, to be tried to "international standards" - whether by the International Criminal Court or in his own country.
And he revealed that fresh sanctions would soon be imposed on Iran amid fears the country is seeking to build nuclear weapons. He refusing to rule out military action but said: "That is not what we are pushing for."