F-16 aircraft costing US$658 million and already paid for by Pakistan were not delivered. Unable to purchase American or NATO weaponry, Pakistan tried to develop a domestic weapons industry, which yielded some successes such as the development of the Al-Khalid Tank and JF-17 Strike Fighter. Soviet–Afghan War During the Soviet occupation of neighbouring Afghanistan, the alliance between the United States and Pakistan was greatly strengthened as the US needed Pakistan as a staging area from which to send weapons to the Mujahideens who were fighting the Soviets. Apprehensive of the two-front threat to Pakistan from India and from Soviet-occupied Afghanistan, the United States in 1981 offered a military aid package of over $1.5 billion, which included 40 F-16 fighters, 100 M-48 tanks, nearly 200 artillery guns and over 1,000 TOW anti-tank missiles, which considerably enhanced Pakistan's defence capability. During the course of the war, Pakistan experienced several air intrusions by Afghan/Soviet pilots and claimed to have shot down eight of these aircraft over the years while losing one F-16 from its own fleet. The Pakistani military, aided by the US and financed by Saudi Arabia, began helping the Mujahideen in setting up training camps and arming them. US President Jimmy Carter had accepted the view that the Soviet aggression was a potential threat to the Persian Gulf region. The uncertain scope of the final objective of Moscow in its sudden southward plunge made the American stake in an independent Pakistan all the more important. Pakistan's ISI and Special Service Group (SSG) became actively involved in the conflict against the Soviets. The SSG created a unit called the Black Storks who were SSG men dressed as Afghan Mujahideen during the Soviet–Afghan War. They were then flown into Afghanistan and provided the Mujahideen with support. After Ronald Reagan was elected in 1980, US aid for the Mujahideen through Pakistan significantly increased. In retaliation, the KHAD, under Afghan President Mohammad Najibullah, carried out (according to the Mitrokhin archives and other sources) a large number of terrorist operations against Pakistan, which also suffered from an influx of weaponry and drugs from Afghanistan. Pakistan took in 3 million Afghan refugees (mostly Pashtun) who were forced to leave their country. Although the refugees were controlled within Pakistan's largest province, Balochistan, then under martial law ruler General Rahimuddin Khan, the influx of so many refugees – believed to be the largest refugee population in the world –