* Keeps up confrontational stance as talks due to resume * sanctions over Iran's nuclear work have hit eco**my * Top leader says the United States can't be trusted (Adds detail, background) By Parisa Hafezi ANKARA, Jan 7 (Reuters) - Iran's top authority Ayatollah Ali Khamenei said on Wednesday Tehran should immunise itself against international sanctions imposed over its nuclear programme, keeping up a confrontational stance as talks over the dispute were due to resume. ...