UNITED NATIONS, OCT. 15 -- The Security Council gave Secretary General Javier Perez de Cuellar new guidelines today for negotiating an end to the war between Iran and Iraq.

The new text offers Iran a concession by suggesting that "the observance of a cease-fire" should start concurrently with the "setting into motion" of a commission to determine who was to blame for starting the fighting seven years ago.

Perez de Cuellar announced afterward that he would present both sides with a "detailed plan" for putting into effect the terms of the cease-fire resolution adopted by the council July 20. He called in the Iranian and Iraqi ambassadors and told them he expects a written reply from each government by the end of the month -- after which he hopes to enter intense negotiations.

The negotiating instructions agreed on by the 15 council members, after three weeks of discussion, are vague and subject to differing interpretations. Apart from linking the cease-fire with the fault-finding commission, as the Soviet Union had publicly proposed, they simply reaffirmed that all parts of the July resolution must be put into effect -- including eventual return to international frontiers, prisoner exchanges and a discussion of who should pay for reconstruction.

Three weeks ago, American officials had said that the foreign ministers of the council's five permanent members -- Britain, China, France, the Soviet Union and United States -- had reached an "understanding" that the instructions for Perez de Cuellar would be accompanied by a negotiating deadline. They said the five nations had also agreed to endorse an arms embargo that would be imposed on whichever side failed to accept the terms of the July resolution.

Iraq has accepted the resolution as a whole, but Iran has hedged on both the timing of the cease-fire and on acceptance of withdrawal to international frontiers.

However, Soviet and Chinese officials said they had a different interpretation of the five-power ministerial meeting. As a result, there was no deadline agreed on in later meetings of the ambassadors of the five powers, and there was no discussion of an arms embargo, participants said.

American officials, too, conceded that Perez de Cuellar achieved no more leverage through today's negotiating instructions than he had in last month's stalemated round of talks in Tehran and Baghdad.