President Reagan, laying down a firm line on arms control negotiations, said yesterday that deployment of American Pershing II and cruise missiles in western Europe beginning this December is necessary to prod the Soviet Union into serious negotiations on reducing medium-range nuclear weapons.

"I think once they see that we and our allies are determined to go forward with the deployment of these weapons, then I think they might meet us in legitimate negotiations," Reagan said in an Oval Office interview with correspondents from the six nations whose leaders will be joining him at the Williamsburg summit this weekend.

The president's comments could reduce the reported expectations of West German Chancellor Helmut Kohl that he can obtain new assurances of arms control flexibility from Reagan when the two leaders confer at Williamsburg.

West German sources said earlier this week that Kohl wants to carry assurances of U.S. flexibility to Moscow on July 4 when he is scheduled to meet with Soviet leader Yuri V. Andropov, in hopes of helping to achieve a U.S.-Soviet agreement before deployment of the Pershing II is scheduled to begin in West Germany.

Reagan gave the foreign journalists yesterday his most blunt, public explanation of the widespread view in the administration that the Soviets will never negotiate on reducing their force of medium-range missiles targeted on western Europe until deployment of the new U.S. missiles begins in West Germany, Britain and Italy.

He told them U.S. deployment "on schedule . . . will be the thing that will bring the Soviets legitimately into negotiations."

In the interview, in which Reagan suggested that Soviet conduct in Afghanistan was comparable to that of Nazi Germany, the president also was pessimistic about the prospects of having a summit meeting with Andropov.

"There has been no evidence on Mr. Andropov's part that he is ready for such a meeting," Reagan said. "He is engaged . . . settling himself into his new position, and I think just to have a summit meeting which might raise the hopes of people all over the world simply to get acquainted and then nothing positive comes from it, makes no sense."

Reagan went on to say that such summits had been tried before "with bad results for our own country and for the world." He said he would be willing to meet with Andropov "when there is an agenda and when there are legitimate issues that could be resolved to the benefit of all of us worldwide."

The interview with journalists from West Germany, Britain, France, Italy, Canada and Japan was set up for discussion of the economic issues to be presented at Williamburg. But their questions demonstrated the preoccupation of Europeans with arms control and, particularly, West German concerns about the Reagan administration's doubts that an agreement can be reached with the Soviets in time to prevent deployment of the Pershing II.

American journalists were not allowed to listen to the interview, which was televised live to Canada and Japan and taped by ABC for use on the evening news. Reagan aides said an interview today with seven U.S. reporters also will not be broadcast in the White House briefing room.

The practice was discontinued after The Washington Post reported May 5 that some reporters listening in the press room joked about some of the president's responses in another group interview in the Oval Office.

In addition to the rare interview for foreign journalists, Reagan met yesterday with Italy's caretaker prime minister, Amintore Fanfani, who said his country would go ahead with planned deployment of the new U.S. nuclear missiles but urged an effort to gain an agreement with the Soviet Union beforehand.

"All European countries agree that we must make the maximum effort for an interim agreement on the missiles," Fanfani told U.S. reporters.

At one point during yesterday's interview, Reagan was asked by one of the foreign journalists whether a combination of the negotiations with the Soviets on intermediate-range nuclear missiles (INF) in Europe and the strategic arms reduction talks (START) on intercontinental nuclear weapons would not be "a way out of this deadlock" on arms control.

"Oh, this is what we hope very much, but in the meantime, what we and the allies must maintain is a deterrent," Reagan replied. "I don't think that any one of us ever contemplates a first strike or making war, and we don't want the other side to have a first strike or make war. Therefore, what we must maintain is not necessarily superiority, but enough force in which the results of a first strike would result in unacceptable damage to the Soviet Union and its allies."

Reagan was asked about a quote attributed to Canadian Prime Minister Pierre Trudeau, defending the testing of U.S. cruise missiles over Canada but complaining that some statements by Reagan and his aides had aroused fears about his commitment to peace.

After thanking Trudeau for his support for the cruise missile testing and suggesting that the Canadian leader had been quoted out of context, the president expressed his commitment to long-term negotiations "to arrive at a real legitimate reduction of these nuclear strategic weapons . . . ," and said he would "persist in this and try for this with every means at my disposal."

But Reagan then launched into a vigorous and emotional criticism of Soviet conduct in world affairs, criticism that he said he would not abandon.

"So I can't at the same time ignore, nor should the civilized world ignore the conduct of a country that today is bombing helpless women and children, is using chemical warfare in places like Kampuchea and Afghanistan," Reagan said.

"I don't think that we can remain silent as too many of us in the world did when Hitler was coming to power, in the face of this kind of conduct. I don't know whether your television networks carried the programs that we saw one day of Soviet soldiers being interviewed in Afghanistan . . . that have deserted and gone over to the Afghan side. And in every incidence, when they were asked why they deserted, they said, 'Because we were ordered to kill women and children.'

"And if I speak frankly about those things, it's because I believe that we in the western world, in the free world, must make it clear that, yes, we want peace and, yes, we're willing to sit down and work out agreements with the Soviet Union, but we want them to know that we're not going to forsake our principles that are based on a love of humanity in order to achieve this."

The only response Reagan gave yesterday in which he sounded conciliatory occurred when he tried to blunt concern about U.S. opposition to East-West trade. Citing his proposal for a long-term grain agreement with the Soviets, he said there is "a misperception that we're interested in some kind of a trade war with the Soviet Union, and we're not at all."