President Reagan told Congress yesterday that the Marines are going into Beirut for an unspecified, "limited" duration but left Capitol Hill no say in how long they will stay there.

The president's letter, required under the War Powers Act whenever U.S. troops are sent abroad, came against a backdrop of congressional unease over the latest Beirut mission and government reports that the open-ended assignment for the Marines is part of a longer-range strategy to keep pressure on Israel to pull its forces entirely out of Lebanon.

The Marines' day-by-day tactical mission is to help the Lebanese in southernmost Beirut keep the main roads free of troublemakers while the Italian and French forces do the same for the rest of the city, according to Pentagon officials.

Strategically, however, the administration hopes that this return trip by the three-nation force will keep Beirut peaceful while the new government takes hold, giving Israel and Syria less excuse to keep troops in Lebanon.

Administration officials said President Reagan delayed the Marines' landing from the weekend to yesterday to increase international pressure on the Israeli government to remove its forces from Beirut. With few other points of leverage, Prime Minister Menachem Begin, in this view, was made to appear the major obstacle to restoring order in Lebanon.

The administration will continue to exert such pressure on Begin until Israeli troops are out of all of Lebanon, officials said, even ordering the Marines to keep their distance from Israeli forces south of Beirut to make the point that the Israeli presence has become a lightning rod for trouble.

"It is possible people might take shots at the Israelis," Pentagon spokesman Henry E. Catto Jr. said yesterday in revealing the Marines have been told to try to stay at least one kilometer away from the Israeli forces. "There is a desire to keep Americans and Israelis apart."

The Marines will fan out from the Beirut airport to the north and east, joining the Italians to the north at about the 48th parallel and eventually moving eastward to the presidential palace, according to the Pentagon.

While administration officials continue to stress that the Marines have been sent to Beirut to help create a peaceful environment, they will not be just a rear guard, as they were for Palestine Liberation Organization combatants leaving the port of Beirut during the first mission.

The Marines will be guarding the corridors leading to the heart of Beirut and the Palestinian refugee camps, hoping to stop any militia-like group before it can start trouble.

Pentagon officials stressed that the Marines would be backing up Lebanese security efforts, not taking over. One general said a typical deployment would be for Marines to man machine guns at crucial checkpoints controlled by the Lebanese.

Meanwhile, the administration sought to clarify Reagan's response to a question in his Tuesday night press conference when he said he expected the Marines to be in Beirut until Israeli and Syrian forces are out of Lebanon.

State Department spokesman Alan Romberg said yesterday that the president "was not setting a criterion" for the departure of the Marines, who began to land in Beirut early yesterday morning.

"He was saying what he expects to happen. . . . It would be up to the government of Lebanon to request the Marines to leave, whether forces are gone or not."

Reagan did little to clarify the situation yesterday in his letter to Congress.

" . . . At this time it is not possible to predict the precise duration of the presence of U.S. forces in Beirut, our agreement with the government of Lebanon makes it clear they will be needed only for a limited duration . . . ," the letter said.

By not citing a specific section of the 1973 act, Reagan left open how long he could keep the Marines there. Had he cited the existence or possibility of serious hostilities, he would have been required to pull the Marines out within 60 days unless Congress acted to allow them to remain.

Sens. Charles H. Percy (R-Ill.) and Claiborne Pell (D-R.I.), ranking members of the Senate Foreign Relations Committee, had written to Reagan last week urging him to recognize the hostile environment of Lebanon. But Reagan's letter said, " . . . As was the case with the . . . earlier multinational force, there is no intention or expectation that U.S. armed forces will become involved in hostilities."

And in the House yesterday, Rep. Paul Findley (R-Ill.) admonished the State Department's leading Middle East policymaker, Assistant Secretary of State Nicholas Veliotes: "Surely, it Beirut is not regarded as a safe area where there is no prospect of hostilities."

Veliotes came under sharp questioning before a House Foreign Affairs subcommittee on the duration and conditions of the force's stay in Lebanon and on the U.S. response to the massacre of Palestinians in Beirut.

Pressed by Rep. Lee H. Hamilton (D-Ind.), chairman of the subcommittee on Europe and the Middle East, whether withdrawal of foreign troops was the condition for removal of the Marines, Veliotes replied:

"What he [the president] meant was that the troops would be there for a limited period but he also expressed the policy goal of getting foreign forces out." He added that American forces could be out before foreign troops withdraw if the Lebanese government asks the Marines to leave.

Reagan told reporters during a political trip to Virginia that he expects the Syrians and Israelis to leave Lebanon "sooner than Lebanon will be ready for us" to remove the Marines, a sentiment echoed by French Foreign Minister Claude Cheysson.

Cheysson, speaking to reporters in New York, said that France would keep its troops in Lebanon as long as the Lebanese government wanted and as long as there was an international framework.

Turning to the question of an Israeli pullout, the French foreign minister said that some kind of international presence may be necessary over a long term in southern Lebanon as a guarantee for Israel's northern border.

Romberg and Veliotes stressed that the president included remaining PLO forces in Lebanon in his references to foreign troops. The president did not mention the PLO forces in his Tuesday night press conference.

In his testimony before the House committee, Veliotes went further than any U.S. official had gone previously in implicating specific Christian militia forces in the massacre of the Palestinians, and he also suggested more explicitly that Israel should have known of the potential for serious trouble in the Palestinian camps in Beirut.

"I can't tell you who pulled the trigger," Veliotes said in response to questions, "but it is reliably reported that elements of the Phalange and elements of the Haddad forces were involved."

Veliotes said that the forces of Maj. Saad Haddad, who has operated primarily in southern Lebanon in close alliance with Israel, "dominated one area of the Sabra camp." Haddad has denied any involvement with the massacre and Lebanon's new president, Amin Gemayel, whose family has long controlled the Christian Phalangist forces, said this past weekend that no Phalangists were given orders to go into the camps.

Asked by Rep. Olympia Snowe (R-Maine) whether Israel should have known that an incident like the massacre was possible, Veliotes said he did "not want to prejudge" the Israeli inquest. But he clearly implied that it should have been known by "anyone who had any knowledge of Lebanon" and who planned sending Christian militia forces into the camps. Veliotes noted that the Israeli Cabinet has admitted that it approved sending the militia into the camps to root out PLO guerrillas believed to be there.

Veliotes volunteered a statement on the peaceful character of the camps after the main PLO withdrawal, saying, "I'm unaware of any evidence, any reports, that anyone in those camps shot back. If there was any threat, I'm not aware of it."