New satellite imagery indicate Iran has expanded its uranium conversion site at Isfahan and reinforced its Natanz underground uranium enrichment plant against possible military strikes, a U.S. think tank said. The U.N. Security Council, which could consider sanctions on Iran, has called on Tehran to halt enrichment activity and asked U.N. nuclear watchdog chief Mohamed ElBaradei to report on the Iranian response on April 28. But Iran has accelerated nuclear work and stood its ground during a visit by ElBaradei last week.The Institute for Science and International Security said in an email sent to news media with attached commercial satellite photos that Iran has built a new tunnel entrance at Isfahan, where uranium is processed into a feed material for enrichment. There had been just two entry points in February, it said. "This new entrance is indicative of a new underground facility or further expansion of the existing one," said ISIS, led by ex-U.N. arms inspector and nuclear expert David Albright.