U.N. Secretary General Javier Perez de Cuellar has presented Britain and Argentina with "concrete ideas" for a settlement of the Falkland Islands crisis involving a U.N. civilian administration of the disputed islands, diplomatic sources said today.

U.N. spokesman Francois Giuliani, who shied away from the word "mediation" in commenting on the U.N. role, said Perez de Cuellar was presenting his compromise ideas to members of the Security Council in a series of meetings this afternoon. The ideas were options rather than a single, specific U.N. peace plan.

Under the suggested U.N. administration, diplomats here said, an administrator, in charge of a dozen or so officials and some civilian policemen, would "deputize" British and Argentine nationals to serve on the U.N. payroll as part of the team.

The U.N. flag would fly over the islands temporarily, after a phased withdrawal by whatever British or Argentine troops and warships were in the area. A U.N. peace-keeping force could be sent to supervise the withdrawals if necessary.

Negotiations then would proceed under a special representative of the secretary general in much the same way the United Nations presides over the intercommunal talks on Cyprus, the diplomats added.

British Foreign Secretary Francis Pym, who was presented with the U.N. options by Perez de Cuellar yesterday, returned to London today after telling reporters, "I don't exclude a U.N. role." He added, however, that "the time is not ripe for the U.N. to take a role" in mediating the dispute.

Pym, who met with Security Council President Ling Qing of China this morning, also played down a Peruvian peace plan, which, he said, American officials had mentioned in passing on Saturday, but which Argentina had rejected.

"It is up to the Argentines to make the first diplomatic move," he insisted. "They've got to change their minds. They started this action and they've got to go."

With the plan, Perez de Cuellar, a 62-year-old Peruvian diplomat who took over as secretary general only four months ago, is facing his first real test as a mediator.

He has moved more slowly and quietly than was the practice of his predecessor, Kurt Waldheim, insisting that both sides request his help, and seeking endorsement of his involvement by the Security Council.

His proposals would require the endorsement of the Security Council, but that would present no difficulty once Britain and Argentina agree.

The U.N. offer has been stymied by the same dispute that ultimately foiled the American mediation effort of Secretary of State Alexander M. Haig Jr.

Argentina has welcomed a U.N. mediating role, but continues to insist that any deal must include assurances that the final outcome of negotiations will be Argentine sovereignty over the islands.

The British reject this Argentine demand, want a commitment from Buenos Aires to withdrawal before their own commitment to negotiate the crisis, and insist that the talks "take into account the views of the islanders."

Diplomats said the idea of a formal U.N. trusteeship over the islands was ruled out for various reasons.

Perez de Cuellar has resisted intense pressure to be more openly active--and, in view of the American mediating experience, has won some retrospective praise from diplomats for his timing.

His first move, several weeks ago, was to form a staff task force that drew up a list of every mediation or peace-keeping operation ever undertaken by the world body.

The team is headed by Rafeeuddin Ahmed, a Pakistani who was the chief staff man for Waldheim's efforts during the Iranian hostage crisis. The group also includes two highly regarded Americans, George Sherry who helps run the U.N. peace-keeping forces, and James Sutterlin, a former Foreign Service officer who has long drafted political policy papers for the secretary general.