Soviet Foreign Minister Andrie Gromyko said today that U.S. military aid to Pakistan was threatening the balance of forces in southern Asia and indicated that Moscow was prepared to provide additional military assistance to India, Pakistan's neighbor and archrival.

In an unusually sharp attack on the government of President Mohammed Zia ul-Haq, Gromyko asserted that Zia's policies were "inevitabley" drawing Pakistan into collusion with the United States and China, which he said want Pakistan as a "bridgehead against the Soviet Union and the countries of South and Southwest Asia."

Zia's policies have stirred increasing concern in neighboring countries about Pakistan's intentions, Gromyko added.

Diplomats here expressed suprise at the tone of Gromyko's remarks in an article he wrote for Pravda, the Communist Party newspaper. It was expected here that an incessant press campaign against Pakistan would be toned down since Gromyko's deputy, Mikolai Firyubin, is scheduled to visit Islamabad later this month in an apparent effort to persuade Zia to recognize the Soviet-backed government in Afghanistan.

Firyubin will be the first senior Soviet official to visit Pakistan since the Soviet invasion of Afghanistan in December 1979.

Gromyko today placed the situation in souther Asia in the context of inccreased international tensions and particularly "growing tendencies in Sino-American relations toward a demonstrable broadening of military aspects of the Peking-Washington partnership."

He said the recent visit to China by Secretary of State Alexander M. Haig Jr. marked "an escalation of unreasonable policies" that the Soviet Union cannot ignore. Gromyko pointed to "plans to supply China with modern American weapons" and said India undoubtedly has assessed the "dangerous character" of these "provocative maneuvers."

The article appeared to have a double purpose, according to diplomatic observers. It continued pressure on Pakistan while it sought to suggest to India that it need its Soviet connection more than ever.

"The scale of the American militarization of Pakistan will destroy the balance of forces in the region and undermine a process of normalization between India and Pakistan which is difficult enough as it is," Gromyko said, adding that the proposed $3 billion in U.S. aid over a five-year period is far more than Pakistan needs for defense.

Gromyko said the Soviet Union made no secret of the fact that it wanted to see a militarily strong India. "That is why it renders assistance necessary to strengthen India's defense capability. The Soviet Union will take all measures demanded by the course of developments and no one should have any doubt that the Soviet state will be able to defend itself, its allies and its friends."

Western specialists said the proposed U.S. assistance is well below what Pakistan needs to balance India's inceasing military strength.

A year ago India and the Soviet Union concluded a large arms deal that some analysts saw as a reward for India's noncommittal attitude on the Soviet invasion of Afghanistan. The Soviets granted India $1.63 billion on a 17-year credit at 2.5 percent interest.

Under this deal, India would build two major Soviet weapons systems -- the Mig-23 fighter-bonmber and the T72 tank. The Indian Air Force is twice the size of Pakistan's.

Against this background, according to diplomatic observers, Gromyko's article appears to cap to barrage of press criticism ranging from attacks on Pakistani tolerance of Afghan rebels and the charges that Pakistan was supplying the rebels with weapons to the often-repeated assertion that Zia's repprochement with Washington constitutes "the main obstacle" to settling the Afghan problem.