NEW YORK - The United States has told Pakistan not to set AQ Khan free as a former arms inspector said Monday Washington and the UN atomic watchdog must be allowed to question the Pakistani scientist if he sold blueprints for an advanced nuclear weapon to Iran or North Korea. Khan, the father of Pakistan's atomic bomb who was placed under house arrest for overseeing a network that sold nuclear weapons' secrets and equipment, is accused of spreading plans for an advanced nuclear warhead in a new report by former UN arms inspector David Albright, an American. Albright said on CNN there was a danger that Khan might be released without having to answer questions about the sensitive blueprints, which show how to build a warhead compact enough to fit on a ballistic missile. "Khan may be released from house arrest. And we may never get to the bottom of this, he said. "So I think it's very important that we start to put pressure on the governments involved in this to find a way to get to the bottom of it." Meanwhile, The New York Times said in dispatch detailing Albright's report that in recent weeks, American officials have privately warned the new government in Pakistan about the dangers of releasing AQ Khan.  "We've been very direct with them that releasing Khan could cause a world of trouble," an unnamed senior administration official who has been involved in the effort said last week, was quoted as saying by The Times. "The problem with Pakistan these days is that you never know who is making the decision - the army, the intelligence agencies, the President or the new government." In the CNN interview, Albright said it was "imperative" that Khan's associates cooperate with investigators and "Khan needs to be interviewed by the United States, by the International Atomic Agency." According to Albright, the arms inspector, the blueprints dating to 2006 are far more troubling, because they offered instructions for building a coveted compact device. Such information would be extremely valuable for countries with nuclear ambitions such as Iran or North Korea, providing a shortcut to making smaller atomic weapons, Albright said. "After years of work, Pakistan learned to make the nuclear weapons smaller and lighter. And so for many countries like Iran, North Korea, that's really the challenge. "It's not hard to make a nuclear weapon per se if you have the nuclear explosive material but it can be a challenge to make it small enough to fit on the ballistic missiles they have. This is that type," he said. Albright said files found on computers by Swiss authorities prosecuting three members of Khan's network contained information about the compact nuclear warheads. "It looks like Khan did steal them (blueprints) and try to peddle them," he said. Last month, the Swiss government reportedly announced that computer files and other data seized from three men contained detailed plans for building nuclear weapons and that the files were destroyed because they posed a risk to national security and could fall into the wrong hands. Dr Khan is an expert in centrifuges used to produce enriched uranium for bomb fuel, and much of the technology he sold involved enrichment, The Times said. But, the paper said, it was only in recent months that officials have begun to confirm that they had found the electronic design for a bomb itself among material seized from some of Dr Khan's top lieutenants, a Swiss family, the Tinners. The same design documents were found in computers in three other locations connected to Khan operatives, according to a senior foreign diplomat involved in the investigation. The blueprints bear a strong resemblance to weapons tested by Pakistan a decade ago, said two unnamed senior diplomats involved in the investigation, the paper said. Pakistani officials have not provided much information about the newly revealed warhead design, just as they have refused to allow the CIA or international atomic inspectors to directly interrogate Dr Khan. Pakistani officials insist that Dr Khan, as the leader of a uranium enrichment programme, had no weapons access. But this is the second weapons design found in his smuggling network, according to The Times. The first was for an unwieldy but effective Chinese design from the mid-1960s that Libya acknowledged obtaining from the Khan network before it surrendered its bomb-making equipment in 2003. Both the new and the old designs exploit the principle of implosion, in which a blast wave from a sphere of conventional explosives squeezes inward with tremendous force to compress a ball of bomb fuel, starting the chain reaction and the atomic explosion, it said. A nuclear official in Europe familiar with the Khan investigation said the new design was powerful but miniaturised - using about half the uranium fuel of the older design to produce a greater explosive force.  "Pakistan cannot put the big China design on any of its rockets," the unnamed official said. "It's too big." A smaller warhead created from the new design, he added, is "more efficient and easier to hide," meaning that one day it might become a "terrorist issue."