The US was not happy with Pakistan developing nuclear weapons, and attempted to hinder the program in various ways. These were complicated, however, by US dependance on Pakistan as a staging ground and funnel for arms and money during the Soviet-Afghan war, and in general by its dependence on military bases in Pakistan throughout the Cold War and post-Cold War.

In the 1970s, the US tried to intervene several times to keep France from sending sensitive and dual-use nuclear technology to Pakistan. It was moderately successful at this. Pakistan was able to acquire the technology through other means, including the smuggling of centrifuge information from the West European company URENCO by A.Q. Khan.

Ultimately the US tried many half-measures with Pakistan. It attempted to secure a promise from them not to enrich their uranium beyond a certain point. The Pakistanis claimed that they were not building a bomb, but could do so if they wanted to. Pakistan stalled and lied to the Americans numerous times. As they were not a signatory to the Nuclear Non-Proliferation Treaty, there were limited things the US could do, and again, the US was trying to scold Pakistan with one hand while use them for their own military endeavors with the other. Throughout the 1970s and 1980s, the US had very good intelligence about the extent of the Pakistani nuclear efforts, but did nothing more than send letters and occasionally emissaries, which were greeted with a polite denial of any clandestine activities. At the same time the US continued to send Pakistan hundreds of millions in military aid, despite legislation meant to hinder the US's cooperation with Pakistan (as long as the US certified that Pakistan "did not possess a nuclear explosive device," it was OK to send them money, even though the legislation in question was created after several Pakistanis were arrested for attempting to illegally buy nuclear components). Not until 1990 was the US willing to (reluctantly) stop certifying that Pakistan was non-nuclear.

All of which is to say: the US did not encourage the Pakistanis to develop nuclear weapons, and did not directly aid them. It attempted to dissuade and prevent them from getting a nuclear capability. However, because of its own conflict of interests with regards to Pakistan and US interests in the region, it basically let them "off the hook" despite being well-aware of their work. So you might consider that an "assist" of sorts, the sort of thing the US would not do for other nations.

For a good overview of this, there is much on Pakistan in Jeffrey Richelson's Spying on the Bomb.