Secret American intelligence assessments have concluded that Iran has obtained a cache of advanced missiles, based on a Russian design, that are much more powerful than anything Washington has publicly conceded that Tehran has in its arsenal, diplomatic cables show. Iran obtained 19 of the missiles from North Korea, according to a cable dated Feb. 24 of this year. The cable is a detailed, highly classified account of a meeting between top Russian officials and an American delegation led by Vann H. Van Diepen, an official with the State Department’s nonproliferation division who, as a national intelligence officer several years ago, played a crucial role in the 2007 assessment of Iran’s nuclear capacity. The missiles could for the first time give Iran the capacity to strike at capitals in Western Europe or easily reach Moscow, and American officials warned that their advanced propulsion could speed Iran’s development of intercontinental ballistic missiles. There has been scattered but persistent speculation on the topic since 2006, when fragmentary reports surfaced that North Korea might have sold Iran missiles based on a Russian design called the R-27, once used aboard Soviet submarines to carry nuclear warheads. In the unclassified world, many arms control experts concluded that isolated components made their way to Iran, but there has been little support for the idea that complete missiles, with their huge thrusters, had been secretly shipped. The Feb. 24 cable, which is among those obtained by WikiLeaks and made available to a number of news organizations, makes it clear that American intelligence agencies believe that the complete shipment indeed took place, and that Iran is taking pains to master the technology in an attempt to build a new generation of missiles. The missile intelligence also suggests far deeper military — and perhaps nuclear — cooperation between North Korea and Iran than was previously known. At the request of the Obama administration, The New York Times has agreed not to publish the text of the cable. The North Korean version of the advanced missile, known as the BM-25, could carry a nuclear warhead. Many experts say that Iran remains some distance from obtaining a nuclear warhead, especially one small enough to fit atop a missile, though they believe that it has worked hard to do so. Still, the BM-25 would be a significant step up for Iran. Today, the maximum range of Iran’s known ballistic missiles is roughly 1,200 miles, according to experts. That means they could reach targets throughout the Middle East, including Israel, as well as all of Turkey and parts of Eastern Europe. The range of the Russian R-27, launched from a submarine, was said to be up to 1,500 miles. Rocket scientists say the BM-25 is longer and heavier, and carries more fuel, giving it a range of up to 2,000 miles. If fired from Iran, that range, in theory, would let its warheads reach targets as far away as Western Europe, including Berlin. If fired northwestward, the warheads could easily reach Moscow. A range of 2,000 miles is considered medium or intermediate. Traditionally, the United States has defined long-range or intercontinental ballistic missiles as having ranges greater than 3,400 miles. The fuel for the advanced engines goes by the tongue-twisting name of unsymmetrical dimethylhydrazine, according to the secret cables. It is a highly toxic, volatile clear liquid with a sharp, fishy smell. International concern about advances in Iran’s missile program increased last year, after Tehran sent its first satellite into space. Experts said it was clear that the second stage of the rocket, known as the Safir, had employed a new, more powerful class of engines that took advantage of some elements of the Russian technology. American government experts say the engines of the Russian R-27 represent an improvement of roughly 40 percent in lifting force over the kerosene-fired engines that power most Iranian missiles. “Without this higher-energy output, the Safir would have failed in its mission to orbit a small satellite,” said a report issued in May by the International Institute for Strategic Studies, an arms analysis group in London. The London group’s report, though, gives no indication of access to the American intelligence assessment. Indeed, the report argued that while Iran had some elements of the R-27 technology, the available public evidence suggested that it had made no purchase of either the complete North Korean missile or its Russian parent. The cables say that Iran not only obtained the BM-25, but also saw the advanced technology as a way to learn how to design and build a new class of more powerful engines. “Iran wanted engines capable of using more-energetic fuels,” the Feb. 24 cable said, “and buying a batch of BM-25 missiles gives Iran a set it can work on for reverse engineering.” The cable added that Tehran could use the BM-25 technologies as “building blocks” for the production of long-range missiles. But it offered no information to back up that assessment.