The State Department yesterday condemned "unilateral, illegal acts" by Israel in establishing three new Jewish settlements in occupied territory and said the move creates "obstacles to constructive negotiations" in the Middle East.

The brief but strong U.S. reaction, issued after White House clearance, was handed to Israeli Ambassador Simcha Dinitz and announced by State Department spokesman Hodding carter III.

A similar statement and a personal letter from President carter was delivered to Israeli Prime Minister Moshe Dayan in Jerusalem by U.S. Ambassador Samuel Lewis.

In White House conversations with Begin last month, President Carter specifically requested that Israel refrain for the time being from establishing more Jewish enclaves in the predominantly Arab territory on the West Bank of the Jordan River that Israel has occupied since the 1967 war, Begin made no promises, arguing that the land belongs to Israel and can be settled as Israel sees fit.

Despite Carter's requests, which are believed to have been repeated in the current diplomatic exchange, the State Department said yesterday that the United States has obtained no assurances that Israel will refrain from authorizing still more settlements during the drive for an Arab-Israeli peace agreement.

The Israeli embassy here, in a statement by spokesman Aviezer Pazner, rejected the U.S. charge that the settlements are obstacle to negotiations, The embassy said Israel has stated repeatedly its readiness for peace negotiations with the Arabs on all subjects.

Successive U.S. administrations have consistently held that the West Bank land seized from Jordan in 1967 does not belong to Israel but is under temporary military occupation. In this view, Jewish settlements violate international law on the treatment of occupied lands. Moreover, the Israeli settlements raise serious doubt about Israel's willingness to return the occupied land to the Arabs in the context of a peace agreement.

The Associated Press reported from Jerusalem that there are now 79 Israeli settlements with a combined population of about 5,000 Israelis in the territories occupied in 1967. The Arab population in the region is about 1,000,000 the AP said.

The Carter administration in recent weeks has been seeking to avoid confrontations with Israel while trying to get negotiations started toward a comprehensive Arab-Israeli peace agreement. But repeated moves by the Begin government on settlement issues could not be ignored without undermining the basis for a potential agreement with the Arabs, in the view of U.S. officials.

Yesterday's State Department declaration was not notably stronger - nor was it notably milder - than Carter administration statements three weeks ago after Israel announced it was giving legal standing to three existing West Bank settlements. At that time, the President, in a news conference, called Israeli settlements "illegal" and "obstacles to peace," though he added that the obstacles could be overcome.

One difference in the handling of the incidents was that previously the United States issued its condemnation of the Israeli action shortly after learning of it from news reports. This time, the United States delayed a major response for a day, until its ambassador in Israel had presented its views and heard Begin's explanation in a meeting with the Israeli leader.

The State Department, in a separate statement issued yesterday, also criticized a decision by the Israeli cabinet last weekend to extend its administrative regulations to Arabs in the occupied territory. Despite Israel's professed humanitarian aims, the statement said, "the action creates an impression of permanence of Israel's occupation . . . which is not helpful."