By Parisa Hafezi, Louis Charbonneau and Justyna Pawlak GENEVA (Reuters) - Major powers resumed talks on Wednesday on a preliminary agreement to curb Iran's nuclear program with the United States warning it would be "very hard" to clinch a breakthrough deal this week and Tehran flagging "red lines". Each side appeared to tempering anticipation of an imminent agreement after the United States, Russia, China, France, Britain and Germany came close to winning concessions from Tehran in the last round of negotiations two weeks ago. British Foreign Secretary William Hague said remaining differences were narrow in the search for an interim deal that essentially would require Iran to limit its contested uranium enrichment program in exchange for limited relief from sanctions. Russian Foreign Minister Sergei Lavrov said: "We hope the efforts that are being made will be crowned with success at the meeting that opens today in Geneva." A senior U.S. negotiator was more cautious, telling reporters: "I think we can (get a deal).