GENEVA, Jan 15 (Reuters) - Iran can enrich uranium up to 20 percent within four days, its atomic energy chief said on Tuesday, a comment apparently aimed at showing Tehran could quickly expand its enrichment program if its nuclear deal with world powers collapses.Iran’s 2015 accord with world powers caps the level to which it is able to enrich uranium to 3.67 percent purity, well below the 20 percent it was reaching before the deal, and the roughly 90 percent suitable for a nuclear weapon.President Donald Trump pulled the United States out of the deal last May, calling it flawed, and reimposed sanctions on Iran. Tehran refuses to renegotiate and has said the deal could fall apart unless European signatories preserve its economic benefits for the Islamic Republic against U.S. pressure.“If we want to come out of the nuclear deal and produce, within four days we could start our 20 percent,” Ali Salehi, head of the Atomic Energy Organization of Iran, told the semi-official Fars News Agency. “But we already have stockpiles of 20 percent, and the capability.”Salehi did not elaborate on his remark about stockpiles. Iran's reserve of 20 percent enriched uranium was downblended, shipped abroad or turned into fuel plates for a research reactor after the nuclear deal was clinched.Salehi told Reuters in an interview last November that Iran could resume enriching uranium to 20 percent purity - seen as well above the level suitable for fueling civilian power plants - if the 2015 deal's trade spin-offs do not pan out for Tehran.Iran is allowed under the deal to produce nuclear fuel under strict conditions that need to be approved by a working group set up by the signatories. Those conditions include ensuring that the fuel cannot be converted to uranium hexafluoride, the feedstock for centrifuges that enrich uranium.