The Reagan administration called on Israel and the Palestine Liberation Organization yesterday to maintain the increasingly tense cease-fire in Beirut so that "the political dialogue can continue and further bloodshed can be avoided."

A brief statement by State Department spokesman Dean Fischer gave no assessment of the negotiations taking place in Beirut between the PLO and the Lebanese government with U.S. support.

Nonetheless, issuance of the statement with its reference to political dialogue was a sign that Washington still hopes for a negotiated settlement that would bring "an end to the armed Palestinian presence in and around Beirut." This objective was among those the State Department formally endorsed Monday.

Reports from the negotiations reaching official Washington indicated that an agreement in broad principle tentatively had been reached between the PLO and the Lebanese government. However, there was no report of agreement on all-important details of enforcement and timing, sources said.

Officials viewed with skepticism an unofficial report that Fatah, the dominant faction of the PLO, has decided to reject a negotiated solution along the lines being discussed. That is certainly one option open to the Palestinian military command, but no such decision has been communicated through governmental channels, sources said.

Israel has put the United States on notice that the cease-fire is limited, that Israeli forces are prepared to move in a matter of days unless the PLO-Lebanese negotiations bear fruit.

If the negotiations fail or bog down, the administration will face extremely serious choices about its attitude and especially the forcefulness of its views about Israeli attack on the PLO fighters in Beirut.

It is uncertain whether the administration would take strong action to discourage Israel from attacking the PLO, or whether such action would be effective in the short run. On the other hand, U.S. acquiescence in a bloody battle in an Arab capital would have major consequences in the Arab world.

At the White House yesterday, deputy press secretary Larry Speakes said: "We're pleased the cease-fire remains in effect after five days." He declined other comment about the Middle East, saying he was trying to avoid discussing the situation "because of the extreme sensitivity of the delicate negotiations that are going on at the moment."

The administration reportedly has cautioned other governments against public stands at this time because of the potential effect on the talks.