ISTANBUL, JULY 28 -- U.S. Undersecretary of State Michael Armacost, on a mission to appease disgruntled U.S. NATO allies in the eastern Mediterranean region, ended two days of inconclusive talks in Ankara today.

The U.S. special envoy left for neighboring Greece, where the prospects for pacifying Prime Minister Andreas Papandreou are thought to be no more promising than they were with the Turkish government, which traditionally is warmer to Washington's approaches.

At issue in both Turkey and Greece -- historic enemies that provide the eastern anchor for the NATO alliance -- are the terms of their unsettled relationship with the United States and the conditions under which both countries will allow the continuation of U.S. military bases on their soil. Each nation sees Washington as giving too much support to its rival.

While insisting that his trip to this strategic corner of the Mediterranean was for "consultations and not negotiations," Armacost told reporters this afternoon after a final meeting with Prime Minister Turgut Ozal that he had received "no indication" about when the Turkish government might ratify an already negotiated agreement to extend U.S. military and economic commitments in Turkey until 1990.

The initial agreement under which the United States maintains a strategically important network of Navy, Air Force, intelligence and military supply bases in Turkey expired in 1985.

Turkey has refused to ratify the new agreement because of its dissatisfaction with the levels of U.S. military aid and because of what Turkey considers hostile moves in the U.S. Congress to impose "unacceptable conditions" on the use of the aid. Armacost today confirmed press reports here that the Turkish refusal to ratify the agreement has led to the slowdown, or even halt, of some modernization projects on U.S. bases that had been planned in previous agreements.

"It has had some effect in terms of specific projects -- modernizations, upgradings at certain facilities," Armacost told reporters without giving specifics. "Those projects are not moving forward on the schedule that had been anticipated."

Yesterday Turkish Foreign Ministry Undersecretary Nuzhet Kandemir told Armacost that Turkey would not ratify the new agreement until the Reagan administration's military aid requested for Turkey for 1988 is approved by Congress without conditions, and the U.S. government clarifies its position on certain congressional resolutions that Turkey considers hostile.

The Reagan administration had requested more than $900 million in military and economic aid for Turkey for fiscal year 1988.

The requested aid was to support an extensive 10-year modernization program of the Turkish armed forces that the Pentagon has estimated will cost $1.2 billion a year.

Turkish officials and, according to Armacost, the Reagan administration, have been stunned by congressional committee actions that have sought to reduce the latest aid request by up to 40 percent, while attaching conditions that U.S.-supplied weapons cannot be used in Cyprus.

Turkey invaded Cyprus in 1974 when the island's Greek majority staged a coup that threatened to have the ethnically divided but independent island-state merge with Greece. Turkey continues to occupy about 40 percent of the island, where the Turkish population is concentrated.

"Turkey cannot accept conditions and mortgages over its policies," Kandemir told Armacost, according to highly placed Turkish sources.

In Greece, where the current five-year bases agreement with Athens expires at the end of 1988, Armacost faces a more problematic job of fence mending.

Late last month Papandreou suspended the opening of promised preliminary negotiations on a bases renewal agreement.

Having come to power in 1981 on a platform that promised to abolish the U.S. bases in Greece, Papandreou, a Socialist, has since softened his stance with an eye on the monetary benefits provided by the bases to Greece's seriously depressed economy. He has agreed to negotiate a new agreement that would be subject to a national referendum.

But when Washington sought to ask Athens in June about reports that it had had dealings with Palestinian terrorist leader Abu Nidal, Papandreou denounced Washington and said there would be no new bases negotiations until the United States apologized publicly.

Washington has since sent the Greek government a letter of explanation.