DESPITE ITS periodic denials, Pakistan is obviously continuing to work on nuclear weapons technology. Pakistan's ambitions have raised concerns for some time among those governments that try to enforce the rules against nuclear proliferation. Last month, The Wall Street Journal reports, President Reagan wrote a personal letter to Mohammed Zia ul-Haq, the president of Pakistan, expressing his "deep concern" that Pakistan's pursuit of these weapons could undermine relations between the two countries. That's an important and useful declaration. Through most of the past four years, the White House has not visibly paid a great deal of attention to the dangers of proliferation. Mr. Reagan's letter is particularly welcome as an indication of increased interest in a kind of peace-keeping in which American leadership is crucial.

Of all the countries that are actively seeking nuclear arms, Pakistan presents the most complex questions to American diplomacy. The guerrillas fighting the Soviet occupation in Afghanistan depend on an open border to the south and, in the absence of at least tacit Pakistani cooperation, the prospects for the Afghan resistance movement would be poor. To the east, India's nuclear capability has become an incitement to Pakistan almost to the point of obsession.

Beyond that, there is the relationship between Pakistan and China. With great fanfare President Reagan signed a nuclear cooperation agreement with China during his visit there last spring, but he has not yet sent the agreement to Congress for approval. The administration has never offered an explanation for this long delay, but the reasons evidently involve the accusations that the Chinese have been helping Pakistan develop weapons. Since the Chinese vigorously deny it, Mr. Reagan's handling of the nuclear agreement has become a central issue between the two governments. Perhaps Mr. Reagan will submit the agreement to Congress early next year, but that will require the administration to discuss Sino-Pakistani nuclear cooperation at length.

Over the decades, many countries have considered an attempt to build nuclear weapons, and all but a few have abandoned the idea. Most have concluded, after careful thought, that they can protect themselves better through reliable alliances and friendships throughout the world. They have usually decided that the possession of these weapons would not leave them safer, but much less safe. Mr. Reagan has now warned Pakistan that its attempts to gain nuclear arms will inevitably affect the ties between its government and the United States, its most powerful friend and supporter. His letter is a contribution to the world's security and, whether they acknowledge it or not, it is an effort to strengthen the Pakistanis' security in particular.