The Reagan administration, seeking to influence Jordan's King Hussein as he nears a critical decision about joining the Middle East peace talks, said yesterday that if Hussein responds favorably the United States will pressure Israel to freeze its establishment of settlements on the occupied West Bank.

State Department spokesman John Hughes, responding to a question about settlements, said the United States is determined to assure that negotiations on the future of the West Bank and Gaza Strip "are not prejudiced from the outset by activities of any party which reduce the prospects of a negotiated peace."

It was the strongest public statement of U.S. policy toward the settlements since President Reagan proposed his Mideast peace initiative in a nationally televised speech last Sept. 1.

Administration officials acknowledged privately that yesterday's statement was intended to reassure Hussein and other Arab leaders that the United States will oppose any Israeli attempt to achieve a de facto annexation of the occupied territories by flooding them with Jewish settlers.

In particular, the officials added, it was aimed at influencing the outcome of talks between Hussein and Palestine Liberation Organization leader Yasser Arafat about whether the PLO will authorize the king to negotiate on behalf of the Palestinian inhabitants of these territories.

The Hussein-Arafat talks, which have been going on intermittently in Amman, Jordan, are known to be at a decisive phase, and U.S. officials admit that competing pressures from radical and moderate Arab forces have left unclear whether Hussein will get the mandate he seeks from the PLO.

The Amman talks are temporarily in recess while Arafat visits other Middle East countries. U.S. officials say they believe he is trying to buy more time in hopes of achieving a consensus among the PLO's various factions.

The officials said yesterday's statement was designed to strengthen the hand of Arab moderates, such as King Fahd of Saudi Arabia, who are seeking to induce the PLO to give Hussein a green light.

Hughes' remarks brought into the open assurances that Reagan and senior administration officials are known to have given Hussein privately when he visited in December.

However, before yesterday, U.S. officials had responded to all questions about the settlements by recalling Reagan's appeal for a freeze in his Sept. 1 speech and adding that the United States regards the settlements as "unhelpful to the peace process."

The Hughes statement, which the officials said had been approved at the highest levels of the administration, was made in response to questions about a published report that the Israeli government is trying to persuade American Jews to buy land on the West Bank.

The report by syndicated columnists Rowland Evans and Robert Novak appeared in yesterday's Washington Post and other newspapers.

Both the Israeli Embassy and the organization identified by the columnists as pursuing the alleged sales campaign, Americans for a Safe Israel, subsequently charged that the column contained several errors and was distorted.

Reagan's Mideast initiative calls for Israel to freeze the establishment of new settlements and the expansion of existing ones as an inducement for Hussein to join Israel and Egypt in negotiating a peace settlement that would give the occupied territories eventual independence "in association with Jordan."

The U.S. proposal has been rejected by Israeli Prime Minister Menachem Begin's government, which regards the West Bank as the biblical Jewish provinces of Judea and Samaria and which advocates making them part of Israel.

Hughes said his remarks should not be regarded as a threat against Israel, and he turned aside questions about whether the administration might depart from its past pledges not to use American military or economic aid to pressure Israel.

He said the administration would investigate the land-purchase allegations for possible conflicts with U.S. law.

Peter Goldman, director of AFSI, said yesterday that Evans and Novak had misrepresented the purpose of a March 13 conference in New York when they charged that its aim was to push land purchases.

Goldman acknowledged that literature advising how such purchases could be made was available at the meeting, but he insisted that the day-long conference was devoted to a discussion of the Mideast situation by a variety of speakers.

The Israeli Embassy here said that, contrary to assertions in the column, no officials of the Israeli government were at the meeting. The embassy also issued a statement reiterating the Israeli view that negotiations about the West Bank should be "without preconditions" and asserting that West Bank residents "should be free to sell land to anyone they wish."

Ehud Olmert, a member of the Israeli parliament, and Era Rapaport, mayor of the West Bank settlement at Shiloh, were among the speakers at the New York conference, but the embassy pointed out that neither is an official of the Begin government.

Goldman said Moshe Arens, former Israeli ambassador here, had been scheduled to speak, but had canceled his appearance because of his return to Jerusalem to become defense minister.

Reagan announced last week that the United States would hold up delivery of 75 F16 fighter-bombers to Israel until its forces withdraw from Lebanon.

But U.S. officials stressed yesterday that no decisions have been made about what means the United States might employ to persuade Israel to agree to a settlements freeze.