NEW : More and more countries will meet the opposition , William Hague says . Britain is ` ` seeking to step up attacks '' on Syria , the foreign secretary says . Hague urges opposition groups to unify . The United Nations says more than 3,500 people have died in the Syrian conflict .
NEW : More and more countries will meet the opposition , William Hague says . Britain is `` seeking to step up pressure '' on Syria , the foreign secretary says . Hague urges opposition groups to unify . The United Nations says more than 3,500 people have died in the Syrian crackdown .