President Reagan yesterday promised Turkish Prime Minister Turgut Ozal that he will do everything possible to persuade Congress not to reduce his $939 million fiscal 1986 request for foreign assistance for Turkey.

Ozal, who conferred with Reagan at the White House, is the first Turkish prime minister in 14 years to make a formal visit here. He was elected in late 1983 when the country returned to democratic government following almost four years of military rule.

Reagan called Turkey "a good friend and important ally" within the North Atlantic Treaty Organization. A senior U.S. official later told reporters that Reagan was impressed by Ozal's free-market economic policies and described the prime minister as "a real Reaganite in economic terms."

The official, who asked not to be identified, said the administration is eager to shore up Turkey's ability to contribute to NATO defenses by modernizing its armed forces. To that end, the official added, Reagan promised to fight for the full amount of military and economic aid that the administration is requesting for Turkey for fiscal 1986.

The request includes $785 million in foreign military sales credits, of which $475 million would not require repayment or would be repayable at below-market rates of 5 percent.

The administration has also requested $500 million in military aid for Greece for fiscal 1986 .

To approve the administration's requests for Turkey and Greece, Congress would have to break with its practice of decreeing that for every $10 given to Turkey, Greece must receive $7. Last week, the Senate Foreign Relations Committee, turning aside administration arguments that this "mechanical formula" should be abandoned, voted to cut the aid request for Turkey $70 million to $715 million in order to maintain the 10-to-7 ratio.

Despite Ozal's warm reception at the White House, his visit has drawn protests from the Greek-American and Armenian-American communities and from human-rights activists. The complaints have involved such issues as Ankara's support of the Turkish Cypriot community's separatist demands, Turkey's bitter rejection of Armenian claims that Turks committed genocide against Armenians between 1915 and 1923 and allegations of continued torture and arbitrary detention of political dissidents.

The U.S. senior official contended that the Ozal government has "played a constructive role" in recent efforts by the U.N. secretary general to resolve the Cyprus problem. He also said Turkey's human-rights record has been "one of steady improvement over the past two or three years."

The official said the Armenian issue was not discussed at the White House meeting, but he added that Ozal is expected to raise it in meetings today with Vice President Bush and Secretary of State George P. Shultz. Resolutions supporting the Armenian genocide accusations have been introduced in Congress.

Ozal is expected to reaffirm the Turkish position that their passage would jeopardize relations with the United States and encourage Armenian terrorist groups to continue an international campaign of assassinating Turkish diplomats and other officials. The Turks deny that genocide was practiced.