Morocco announced at the General Assembly's 40th anniversary ceremonies today that it was observing a unilateral cease-fire in Western Sahara, starting immediately. It also offered to hold a referendum on self-determination for the territory in early January under U.N. auspices and control.

A ranking representative of the Polisario, the movement fighting for independence of the former Spanish territory, rejected the offer as an "intransigent repetition of known Moroccan positions." The territory was occupied and annexed by Morocco after Spain withdrew 10 years ago. Fighting there has been sporadic in recent years.

Moroccan Prime Minister Mohammed Karim Lamrani, reading a message to the United Nations from King Hassan II, told the assembly that Morocco would welcome neutral on-site observers to verify the cease-fire. He said Moroccan troops would only respond in self-defense "in the case of aggression."

But there was no sign of accommodation with the Polisario. The statement said Morocco refused direct negotiations with "a handful of mercenaries" who were seeking to achieve "the legitimacy and the credibility which they cannot achieve through the referendum."

Ould Salek, the Polisario's chief of external relations, told reporters that Morocco's proposal was designed to "gain time" and warned that "peace will only return with the return of the invasion troops to Morocco before the conclusion of an agreement."

In another speech, Pakistani President Mohammed Zia ul-Haq sought to ease international fears that his country was developing nuclear arms and affirmed Pakistan's "irrevocable commitment not to acquire nuclear weapons or nuclear explosive devices."

He proposed simultaneous pledges by India and Pakistan to sign the treaty barring the spread of nuclear weapons, accept full international safeguards and inspections of nuclear facilities and arrange mutual inspection of each other's atomic installations.In another speech to the assembly, Japanese Prime Minister Yasuhiro Nakasone urged the United States and Soviet Union to "present clearly to all the peoples of the world the actions they intend to take to reduce drastically their nuclear arsenals."

Nakasone noted also that at the time of the United Nations' birth in 1945, "Japan was waging a desperate and lonely war against over 40 allied countries." Since then, Nakasone said, "Japan has profoundly regretted the ultranationalism and militarism it unleashed and the untold suffering the war inflicted upon peoples round the world and, indeed, upon its own people."

Many of the speakers devoted their attention to ways in which the organization could function more effectively as a forum for resolving disputes.

Mexican Foreign Minister Bernardo Sepulveda Amor urged U.N. Secretary General Javier Perez de Cuellar to submit to the Security Council specific proposals for defusing regional conflicts in Central America, the Middle East, Cyprus and Southeast Asia.

Sepulveda also obliquely criticized the United States for withdrawing from the compulsory jursidiction of the World Court in political disputes and accused all major powers of blocking U.N. action by excessive use of their vetoes in the Security Council.