NEW : more than 3,500 people have died in violence Sunday , an activist group says . The United Nations could impose economic sanctions Thursday if Syria ignores deadline . Syria is under international pressure to halt its crackdown on pro-democracy protesters . The United Nations says At least 35 have died since the protests began in March .
NEW : At least 35 people have died in violence Thursday , an activist group says . The Arab League could impose economic sanctions Sunday if Syria ignores deadline . Syria is under international pressure to halt its crackdown on pro-democracy protesters . The United Nations says more than 3,500 have died since the protests began in March .