Secretary of State Cyrus R. Vance asked Congress yesterday to repeal the U.S. arms embargo against Turkish withdrawal from Cyprus and threatens instead to disrupt the NATO alliance.

"The time has come to look forward rather than back," Vance said in testimony before the House International Relations Committee. He added:

"Continued maintenance of the embargo would be harmful to U.S. security concerns, harmful to NATO, harmful to our bilateral relations with Turkey and harmful to our role as a potential contributor to a Cyprus settlement."

Vance was backed by Defense Secretary Harold Brown who warned that, unless the U.S. weapons flow is restored, there is a great danger of Turkey's quitting NATO - a step the administration says would cause an unraveling of NATO defense on Mediterranean southeastern flank.

Nationalistic pressures, Brown said, would cause Turkey "to move away from the United States" and seek other sources of arms. Turkey has traditionally shield away from the Soviet Union, but asserted there is aposibility that Turkey eventually would turn to Moscow as a weapon supplier.

The two cabinet officers testifiedd at the kickoff of the Carter administration's effort to win repeal of the three-year-old embargo - an effort facing strong oppsition from the Greek-American community and its supporters in Congress.

These supporters pushed through sion of Cyprus. Turkish forces continue to occupy roughly 40 percent of the embargo after Turkey's 1974 inva the island country, which has an 80 percent Greek population.

Although the embargo later was modified to permit Turkish arms purchases of up to $175 million a year, it has called a steady deterioration of relations between Washington and Ankara. In retaliation, Turkey closed most of the U.S. bases inside its borders and placed heavy restrictions on these that remain.

Tension escalated after Bulent Ecevit became Turkish prime minister in January. He has threatened to close the U.S. bases permanently and loosen, or sever, Turkey's ties to NATO.

Ecevit's pressures apparently had considerable impact on the Carter administration, which previously had linked relaxation of the embargo to Turkish moves toward a Cyprus settlement. Carter called for such linkage during his 1976 presidential campaign, and, as Vance was reminded yesterday, he too appeared before the International Relations Committee as a private citizen in 1975 to endrose the embargo.

However, the administration abruptly changed course after a visit to Ankara by Deputy Secretary of State Warren M. Christopher last week. The administration then announced that it would seek to end the embargo.

As a gesture to the embargo's congressional supporters, the administration did say it was temporarily dropping plans for a four-year defense co-operation agreement that would have given Turkey $250 million a year in military aid.

Vance said yesterday that, pending a future renegotiation of the defense pact, the administration wants Congress to authorize Turkish military sales credits of $175 million during the 1979 fiscal year. He added that the administration also will ask Congress to approve a $50 million loan to assist the Turkish economy.

In what appeared to be a conciliatory gesture toward Greece's supporters, Vance said the administration was raising its fiscal 1979 military sales credits request for Greece from $122 million to $140 million. Pro-Greece members of Congress had sharply criticized the administration's earlier plan to cut back Greek military aid while giving Turkey the maximum permissible amount.

Vanve told the committee that the embargo failed to achieve its purpose of prodding Turkey to withdraw from Cyprus and negotiate a settlement of the issue that have long divided the island's Greek and Turkish populations.

The embargo, he said, had demonstrated the seriousness withe which the United States regards Turkeys's alleged illegal use of U.S. supplied weapons. But, Vance added, to keep it in effect would only cause "a continued, perhaps even irreversible, deterioration" of U.S. Turkish relations.

Vance said the United States is still very anxious for a Cyprus settlement. The administration, he noted, has concluded that the chances of reaching such a settlement can now be pursued more successfully in an atomsphere of improved relations with Turkey rather than under conditions of continued strain.

In that connection, Vance said, Turkey and its Turkish-Cypriot clients plan to present proposals for a comprehensive Cyprus solution within 10 days. However, he warned, it would be a "serious mistake" for Congress to reserve action on the embargo until it has a clear idea of the Turkish proposals and where they might lead.

At the hearing, Vance also was asked about his finding, in response to questions from Congress, that Israel may have violated U.S. arms in its invasion of southern Lebanon.

He replied that he had no doubt Israel had acted in self-defense and that the administration considered the matter closed because of Israel's promise to withdraw. But, Vance added, the facts of the case still pointed to the conclusion that Israel may have violated the terms of its arms agreement with the United States.