and the strategic importance of Pakistan to the United States was gone. Once the full extent of Pakistan's nuclear weapons development was revealed, economic sanctions (see Pressler amendment) were imposed on the country by several other countries, particularly the US. Having been developed under both Bhutto and Zia, the nuclear development programme had fully matured by the late 1980s. Abdul Qadeer Khan, a metallurgical engineer, greatly contributed to the uranium enrichment programme under both governments. A Q Khan established an administrative proliferation network through Dubai to smuggle URENCO nuclear technology to Khan Research Laboratories. He then established Pakistan's gas-centrifuge program based on the URENCO's Zippe-type centrifuge. Khan is considered to be the founder of Pakistan's HEU-based gas-centrifuge uranium enrichment programme, which was originally launched by PAEC in 1974. The PAEC also played its part in the success and development of the uranium-enrichment programme by producing the uranium hexafluoride gas feedstock for enrichment. PAEC was also responsible for all the pre- and post-enrichment phases of the nuclear fuel cycle. By 1986 PAEC Chairman Munir Ahmad Khan had begun work on the 50 MW plutonium and tritium production reactor at Khushab, known as Khushab Reactor Complex, which became operational by 1998. After India succeeded with five underground nuclear tests (codename Pokharan-II) in Pokhran region in 1998, Pakistan, to the distaste of the international community, successfully carried out six underground nuclear tests in Ras Koh region of the Chagai Hills on May 28 (codename Chagai-I) and on Kharan region (codename Chagai-II) on May 30, proving Pakistan's nuclear capability. These tests were supervised and observed by physicist Samar Mubarakmand and other senior academic scientists from PAEC and the KRL. US sanctions US Senator Pressler introduced the Pressler Amendment, which imposed an embargo on all economic and military aid to Pakistan for developing nuclear weapons. This caused very negative publicity in Pakistan towards the US as many people in Pakistan, and particularly the Pakistani Armed Forces, believed they had been abandoned after they risked a great deal in helping thwart the Soviet Union in Afghanistan. Pakistan was hosting a very large Afghan refugee population and drugs from Afghanistan had infiltrated Pakistan where the use of heroin was growing into a widespread problem. The embargo continued for five years and in 1995, the Brown Amendment authorised a one-time delivery of US military equipment, contracted for prior to October 1990, worth US$368 million. However, the additional 28