President Mohammed Zia ul-Haq of Pakistan left Tehran for Baghdad yesterday on a peace mission that failed to evoke any positive response from Iran's leaders, who were reported to have rejected out of hand the Islamic nations' attempt to mediate the dispute.

After two meetings with Iranian President Abol Hassan Bani-Sadr, Zia said before leaving that "Iran is still in a revolutionary stage . . . when conciliation and mediation do not work." But he said he hoped "sanity will prevail" eventually and that both sides would realize that a continuation of the war was in neither side's interest.

An aide to Bani-Sadr was even more blunt about Zia's abortive talks in Tehran, saying, "He achieved nothing. It was nice of him to come but we are going to fight on until the last Iraqi soldier is off Iranian soil."

As indications mounted that Zia's mission was heading for failure, Pakistan withdrew its objections to a substantive debate at the United Nations Security Council on the Iraqi-Iranian war. Pakistan, which is leading the peace effort on behalf of the 40-member Conference of Islamic Nations, earlier had opposed the debate fearing it might interfere with Zia's mission.

By late afternoon, the council unanamously adopted a resolution calling on Iran and Iraq to end the war and accept help in settling their differences.

The Mexican-sponsored measure asked other countries to avoid widening the conflict and backed Secretary General Kurt Waldheim's offer to help mediate. Waldheim was asked to report back to the council in 48 hours on his efforts.

But there was slim hope that either Iran or Iraq was ready yet to abide by such a resolution, and the fighting raged on through its seventh day with no sign of letup.

Meanwhile, U.S. Deputy Secretary of State Warren Christopher expresed concern that additional actions and reactions by the two Persian Gulf nations could bring greater danger to all concerned.

Speaking on the CBS "Face the Nation" program, Christopher called the war a "classic case of escalation" that threatened to get out of control. "What began as a ground thrust escalated into air exchanges, that escalated into air attacks on civilian targets and economic targets."

He said the United States was doing all it could through diplomatic means to contribute to a settlement, but that neither Iran nor Iraq showed any desire at present for a cease-fire. He said there was no clear indication how long the war might continue.

In Baghdad, Iraqi Foreign Minister Saadoun Hammadi told a news conference Saturday that Iraq would not consider a cease-fire until the Iranians "come to their senses" and agree to accept its sovereignty over the disputed Shatt-al-Arab waterway and other territory at issue.

Another private initiative under way to mediate the dispute also seemed headed for failure. Palestine Liberation Organization chief Yasser Arafat met with Iranian leaders but appeared to have irritated them by not coming out on their side in the conflict.

"We know Arafat has a special affection for the Iranian revolution, but he has to stop playing around. His position is not very clear at the moment," Reuter quoted one unnamed Iranian official as saying about the Arafat mission.

Zia met with Arafat before leaving for Amman, Jordan, where the Pakistani leader was scheduled to stay overnight before proceeding to Iraq Monday on the second leg of his peace mission.

In Bulgaria, a Moscow-sponsored Parliament of the Peoples for Peace meeting in Sofia passed a resolution calling on both Iran and Iraq to show "responsibility" and end their fighting.

The Soviet Communist Party newspaper Pravda, reporting on the work of the meeting, said it had also expressed full support for efforts under way at the United Nations aimed at ending the conflict."

This was an early indication that Moscow would support the U.N. call for a cease-fire.

Mounting Western concern about the war caused British Foreign Secretary Lord Carrington to cancel a trip he was planning to China and Japan. He is ready to leave the U.N. General Assembly session in New York for London Monday night.