VIENNA - Iran has begun installing advanced centrifuges at its main uranium enrichment plant, a UN nuclear report said on Thursday. In response, Prime Minister Binyamin Netanyahu said that Iran is closer today than ever before to obtaining the necessary enriched uranium for a nuclear bomb. The White House said that the window remains open for diplomacy with Iran but will not stay open indefinitely. Netanyahu termed the report "very grave," and said it proved that Iran is moving swiftly toward the red line he set out at the UN in September. He said during that address that Iran must be stopped before it crosses that line, something he said at the time could happen as early as the spring.Iran resumed converting higher-grade enriched uranium for fuel production in December and had since fed 28.3 kg of the material for this stated purpose, the report added.It further said that "extensive" activities - an allusion to clean-up and renovations - at Iran's Parchin military site would seriously undermine an IAEA investigation to determine whether explosives research relevant to nuclear weapons was done there.The Prime Minister's Office said that preventing Iran from obtaining nuclear weapons will be the first issue on the agenda when US President Barack Obama comes to visit in less than a month's time.In a confidential report, the International Atomic Energy Agency (IAEA) said 180 so-called IR-2m centrifuges and empty centrifuge casings had been hooked up at the plant near the central town of Natanz.If operated successfully, such machines could enable Iran to significantly speed up its accumulation of material that the West fears could be used to devise a nuclear weapon. Iran says it is refining uranium only for peaceful energy purposes.The report also said Iran had increased to 167 kg (367 pounds) its stockpile of uranium refined to a fissile purity of 20 percent - a level it says it needs for conversion into reactor fuel. About 240-250 kg of 20 percent enriched uranium is needed for one atomic bomb if refined to a high degree.