The United States intends to supply new Sidewinder air-to-air missiles to Pakistan to be fitted on the F16 combat aircraft already delivered to the country, a senior U.S. official said here today.

Michael H. Armacost, U.S. undersecretary of state for political affairs, told a press conference at the conclusion of a four-day visit that Washington was concerned at the frequency in recent months of air raids on Pakistani border areas from Soviet-occupied Afghanistan.

The Reagan administration had notified Congress on March 5 of its intention to supply the missiles to Pakistan.

Armacost identified the new arms as AIM9L missiles, Reuter reported, but would not say how many would be supplied. In Washington, U.S. officials said the weapon was a more recent model of the Sidewinder missiles that have been supplied to Pakistan since the 1960s. Pakistan has now received more than half of the 40 F16s promised, Reuter reported.

"We recognize the pressures imposed on Pakistan by the Soviet occupation with 100,000 troops in Afghanistan," Armacost said. "It is for this reason that we offered the kind of military assistance that would augment the air defense capabilities of Pakistan."

Armacost said he did not agree that reported U.S. covert aid to the Afghan resistance -- which he would neither confirm nor deny -- worked to increase that pressure. The tension arose out of the Soviet invasion of Afghanistan, he said.

After the press conference Armacost left for India, which bitterly opposes the U.S. arms supplies to Pakistan under a $3.2 billion economic and military aid package signed following the Soviet invasion of Afghanistan. India fears the arms might be used against it.

Armacost said he found "deeply heartening" Pakistan's desire to improve relations with India. The United States, too, was looking forward to improving its relations with that country, but "not at the expense of Pakistan," he added.

Armacost also said he felt "heartened" by Pakistan's assurances that it intended neither to acquire nuclear weapons nor to explode a nuclear device, but he said that "from our standpoint we would welcome supplementing of such assurances with verification arrangements through the International Atomic Energy Agency."

Armacost is due to return to Islamabad from New Delhi this weekend on his way to Peking.

Pakistan charged today that Afghan warplanes violated Pakistan's airspace yesterday for the second time in a week when four Afghan jets and two helicopters penetrated about 5 miles over Arandu in the northern section of the border. A government press statement said no casualties or damage resulted, but added that the Afghan charge d'affaires in Islamabad had been called to the Foreign Office and handed a strong protest.