The United States called on the U.N. Security Council tonight to help resolve the Iranian crisis, saying the fate of 50 American hostages in Tehran was "not negotiable" but offering to discuss outstanding issues with Iran once they are freed.

Charging that "no country can break and ignore the law while seeking its benefits," U.S. Ambassador Donald McHenry told an emergency council meeting that any Iranian grievances could be heard "in an appropriate forum."

But, he continued, "the United States insists that its diplomatic personnel be released and its diplomatic premises restored. These are not negotiable matters. The United States will hold the authorities in Iran fully responsible for the safety of the American held captive."

McHenry's carefully worded speech came against a background of deepening pessimism here about the ability of the Security Council to adopt effective steps because Iran, which originally had requested the meeting, has since decided to boycott it and ignore its decisions.

Other members of the council who spoke tonight, including the Soviet Union and China, endorsed the U.S. call for the release of the hostages. The two Communist powers and some Third World countries, while saying that Iran had legitimate grievances that should be heard, insisted that the rules of international law must be respected.

Despite the shadow cast over the proceedings by Iran's defiant stance, the United States has pressed ahead with its efforts to get the 15-nation council on record behind a formal resolution that, at the least, would declare Iran obligated under international law to free the hostages.

The Carter administration wants such a resolution as part of its campaign to keep the dispute focused on the plight of the hostages, to emphasize to Iran that it is isolated in world opinion and to demonstrate that the United States is willing to use every available international forum to find a peaceful solution to the 28-day-old crisis.

The importance that Washington attaches to this effort was underscored by the presence at the Security Council meeting of a congressional delegation headed by Sen. Frank Church (D-Idaho), chairman of the Senate Foreign Relations Committee, and Rep. Clement J. Zablocki (D-Wis.), chairman of the House Foreign Affairs Committee. They said they had come to demonstrate to the U.N. membership that President Carter had broad, bipartisan support for his handling of the Iranian situation and that the people of the United States expect both moral and concrete backing from the United Nations.

"The president, the Congress and the American people have shown great restraint in the face of extraordinary provocation," Church said shortly before the start of the council debate. "But that cannot go on indefinitely. We hope the U.N. can play a role bringing about a peaceful resolution."

"The U.N. itself is on trial as to its effectiveness," Church added. "It's effectiveness, we hope, can be established in the coming days."

McHenry said U.S. hostages Tehran are "being held under degrading conditions. They are threatened, kept bound, isolated, not allowed to speak, denied mail. Even their whereabouts are uncertain.

"The situation in Tehran has an feature unlike other assaults on the diplomatic ties that bind our world," McHenry said. "In Iran, the government itself defends the violence which holds diplomats hostage."

He referred obliquely to Iranian complaints that the United States had cooperated in the past with autocratic regime of deposed Shah Mohammad Reza Pahlavi. Iran took the hostages to back its demands that the shah, who has been hospitalized in New York for treatment of cancer and gallstones, be returned to stand trial as a criminal.

The shah remained secluded in his hospital room today still looking for a nation that will grant him asylum. Most of his personal belongings were removed from the New York Hospital-Cornell Medical Center yesterday.

"None of us is deaf to the passionate voices that speak of injustice, that cry out against past wrongs and that ask for understanding," McHenry said."There is not a single grievance alleged or spoken in this situation that could not be heard in an appropriate forum."

"The United States remains ready, upon the release of the hostages, to discuss with the Iranian authorities the differences which exist between us and to seek their resolution," McHenry said.

"But make no mistake," he said in conclusion. "Beneath that discipline is a seething anger which Americans properly feel as they witness on daily television new threats and outrages against their fellow citizens. The hostages must be freed."

McHenry's speech kicked off a debate which many U.S. observers expect to run for at least two to three more days. In the end, most observers here said privately, Washington probably can expect little more than a relatively bland and generalized resolution calling attention to international agreements for the treatment of diplomats and reiterating past U.N. calls to release the hostages.

While many Third World countries have shown no sympathy for Iran's actions, they also are clearly skittish about explicitly condemning Khomeini's revolutionary government or voting for specific measures such as the imposition of U.N. sanctions against Iran.

That is especially true of Islamic nations fearful of a possible backlash from fundamentalist Moslems among their own citizens if the Security Council bakcs some kind of collective action against Iran or even does anything the United States might use later as a justification for military action.

Such Arab neighbors of Iran as Saudi Arabia, Kuwait, and the United Arab Emirates have signaled Washington that resorting to force or other tough steps could trigger unpredictable waves of violence throughout the Middle East.

The council adjourned shortly before midnight and scheduled its next session at 7:30 p.m. Sunday.

The general expectation here is that any resolution adopted by the council will not go much beyond calling for the release of the hostages and perhaps saying that Iran should obey the decision of the International Court of Justice, which is scheduled to begin hearing the U.S. complaint on Dec. 10. j

As to whether Iran would obey such a resolution, most diplomatic sources here seem either frankly pessimistic or unwilling to speculate. However, one note of optimism was sounded by U.N. Secretary General Kurt Waldheim, who took the initiative in calling the Security Council meeting on Iran.

Waldheim told reporters he had spoken by telephone earlier today with the new Iranian acting foreign minister, Sadegh Ghotbzadeh. Waldheim said that Ghotbzadeh, while reiterating his intention not to come to the United Nations "for the time being," did leave the impression that "there is still considerable interest in Tehran in trying to solve the problem through negotiation."