The United States and the Soviet Union have recently exchanged letters on the future of Afghanistan, but it remains questionable whether an international accord involving the withdrawal of Soviet troops can be arranged, Secretary of State George P. Shultz said here today.

Shultz spoke of his correspondence with Soviet Foreign Minister Andrei Gromyko, which U.S. sources said took place in May, at a news conference on the last afternoon of his visit to India.

Saturday Shultz flies to Pakistan, where the subject of the international negotiations on neighboring Afghanistan and other Afghan-related topics will be at the top of his agenda.

According to officials accompanying Shultz, the letter to Gromyko was prompted by reports that Soviet diplomats in several capitals were saying, publicly and privately, that the United States really does not wish a diplomatic resolution of the Afghan question but is bent on keeping the problem alive as an international pressure point against Moscow and a drain on Soviet resources.

Shultz said the purpose of his letter was "to assure the Soviet Union as we have assured others who are involved that we wish to see it settled." U.S. policy, he added, is to see a settlement based on the U.N. guidelines, which involve withdrawal of Soviet forces, return of refugees, self-determination for the Afghan people and an independent, nonaligned Afghanistan.

He did not characterize Gromyko's response, but other sources said it combined, in very blunt fashion, reiteration of prior Soviet positions with accusations against the United States.

A third round of U.N.-sponsored "indirect talks" between Pakistan and the Soviet-backed Afghan regime has just been completed in Geneva. Pakistani Foreign Minister Yakub Khan, who was informed of Shultz's letter to Gromyko during his May visit to Washington, will brief Shultz on the status of the Geneva talks.

Asked about reports of negotiations between India and U.S. weapons manufacturers for arms sales of nearly $1 billion, Shultz said the matter was discussed during his visit, but added, "Whether or not there will be actual sales is an open question." He said the United States is "prepared to make such sales."

India's reported interest in U.S. arms, coupled with recent purchase orders of Mirage 2000s from France and weaponry contracts with Britain, has been heralded here as a significant move toward diversification of arms sources apart from India's predominant supplier, the Soviet Union.

In response to a report in a West German newspaper, Sueddeutsche Zeitung, that India last week signed a letter of intent for a $5 billion arms deal with the Soviet Union, Shultz said, "I'm not aware of that transaction." He would not comment further.

According to the report, Indian Defense Minister Ramaswami Venkataraman, signed an agreement during a visit to Moscow last week in which India will purchase an unspecified number of MiG29s and produce under license MiG27 fighters as well as spare parts for sale to other countries who have MiGs.

The report also said that India will buy a large number of Ilyushin76 transport planes, helicopter gunships, surface-to-air missiles and 672 tanks and will obtain an option for the next generation of Soviet tanks.

During his Moscow trip, Venkataraman issued a statement saying that India and the Soviet Union had agreed to expand their military cooperation but no details were offered and no mention has been made about the schedule for any agreement.