ISLAMABAD, PAKISTAN -- Moscow's stealthy decision to build India's military might with nuclear submarines and AWACS-style radar planes is both bad news for Pakistan and highly disturbing for U.S. and Western security beyond the Persian Gulf.

India is ''ambitious'' to become a ''regional superpower'' from the Gulf to the coast of China, President Zia of Pakistan told us over lunch. That ambition makes a snug fit with the long Soviet effort to undermine Pakistan, India's archrival.

Next to Turkey, Pakistan is America's strongest Asian ally, crucial in blocking a Soviet victory in Afghanistan. ''Only two countries in South Asia -- Turkey and Pakistan -- have stood up to the Soviets,'' Zia said. ''Take Pakistan out and the Soviets would become supreme from Turkey to Vietnam.''

Western diplomats here agree with that sentiment. Judging from the U.S. commitments in the Persian Gulf and Afghanistan, so do President Reagan's policy-makers in Washington.

But what is not yet so clear is whether Reagan and his advisers fully understand the potential menace of the tightening Soviet-Indian alliance. The expected addition of three nuclear-powered submarines to the Indian Navy, in addition to a couple of aircraft carriers that are coming from the British, suggests a future challenge to U.S. supremacy in the Indian Ocean. India was furious in 1972 when, during the celebrated ''tilt'' to Pakistan, the United States sent a carrier task force there.

Things have now changed. Indeed, with Indian naval bases on both sides of the subcontinent, even Australia is signaling distress over the potential impact of such a large naval force.

The Soviet fleet, with a base at Aden, another on a key island in the Indian Ocean and a third huge naval arsenal at the U.S.-built Camranh Bay on the coast of South Vietnam, has given nightmares to Pentagon planners ever since the United States was driven off mainland Asia in the Vietnam War. The new Indian Navy now becomes a major add-on to the powerful Soviet fleet.

But India's military acquisitions from Moscow go a good deal further. One wing of MiG-29s, the latest Soviet model, is on its way and at least two more appear to have been promised to India. No Soviet-bloc state has ever been allowed to get its hands on the MiG-29.

As for the AWACS-style early-warning planes, the Soviet promise is ironically tied to Moscow's strategy in Afghanistan to destabilize Pakistan. Official figures show that Soviet-Afghan air attacks across the border into Pakistan killed or wounded at least 966 Pakistani citizens between 1980 and last May. This is the Soviet fist to split Pakistan and the United States, ending Pakistan's aid to Afghan refugees and the mujaheddin freedom-fighters.

But Pakistan has refused to buckle to that pressure. Instead, Zia asked the United States for AWACS aircraft to protect its border. That sent India rushing to Moscow demanding equal treatment, claiming that early-warning planes for Pakistan would be a security risk for India. And what happened? India is getting its planes, while the United States continues to ''study'' Pakistan's request.

The Soviet buildup of India to a regional superpower has not been big news in the United States. Most members of Congress seem uninterested, a fact underlining pro-Indian political prejudice. If Pakistan received a nuclear-powered sub from the Soviets, Congress would cancel all leaves and work through the weekend to handle the resolutions of condemnation and the bills cutting off U.S. aid.

This prejudice of U.S. politicians, despite India's naval buildup, is even more surprising in view of India's support for the Soviet invasion of Afghanistan. Officially, India sees no evil. It accepts the preposterous Soviet claim that the Red Army occupied Afghanistan because of outside ''interference'' with the Communist government -- the very regime that was planted in Kabul by the Kremlin.

India has even informed Moscow that it will give asylum to Afghan Communist leaders if they have to flee for their lives following the oft-promised Soviet withdrawal.

That perverse loyalty to the Soviets by the country that has led the world in preaching against imperialism and for human rights shows how badly India wants to achieve its ambition as regional superpower. The probable targets in this power game are the United States. and Pakistan, a fact well understood by President Zia, whose strategic mind is often underestimated in the United States. Even without the new India, Zia's strategic warning deserves closest attention in Washington.