The Soviet Union today vetoed a Security Council resolution condemning the invasion of Afghanistan and sponsors of the resolution are expected to call Tuesday for an emergency session of the General Assembly on the issue.

The council also took up in a private consultative session a Dec. 31 resolution calling for sanctions against Iran, but delayed public actions. Secretary General Kurt Waldheim presented a report of his trip to Iran and the council ruled that the private session met the Jan. 7 deadline for action imposed in the original resolution.

A spokesman for the U.S. mission here said tonight that a sanctions resolution is likely to be presented "by the end of the week."

Several diplomatic sources indicated that the resolution has been postponed to iron out details in its wording to the satisfaction of the Western allies, who have a significant dependence on Iranian oil.

At the same time, administration sources said, a number of sympathetic Third World nations have advised the United States to "take advantage" of widespread anti-Soviet feeling on the Afghanistan issue and not "muddy the waters" by pushing a vote on Iran.

The resolution condemning the Afghan invasion did not mention the Soviet Union by name. The vote for the measure was 13 to 2.

Although the original Dec. 31 resolution on Iran passed by an 11-to-0 vote, with the rest of the council abstaining, several of the Third World nations that supported it have indicated that they may not approve sanctions.

The U.S. mission tonight privately circulated copies of its proposed sanctions resolution among council delegations. The resolution is believed to call for a full-scale embargo of all exports to Iran except food and medicine. c

Third World nations are reluctant to approve a sanctions resolution against one of their own, and a number of Western European governments privately have voiced reservations about the scope and terms of the sanctions.

Sources said the British, in particular, are concerned about provisions calling for retroactive implementation of the sanctions that would involve previously committed lines of credit or contracts.

In an interview tonight, U.S. Ambassador Donald McHenry insisted that "everything is on schedule, regarding the sanctions resolution. "It is in the interest of the council to engage in consultations during the next several days," he said. "The council will meet and will act before the week is out.

"The United States' intention is to go in accordance with the resolution Dec. 31. We are discussing some specifics and hope to have them nailed out within the next several days."

In other action today, the General Assembly overwhelmingly approved Mexico's last-minute bid to occupy a vacancy on the Security Council. Mexico was nominated this morning by the Latin American regional group following the withdrawal of Cuba and Colombia, whose contention for the chair had deadlocked the assembly since October.

Mexico joined a 13-member majority late this afternoon in approving the resolution "deploring the recent armed intervention in Afghanistan" and calling for "the immediate and unconditional withdrawal of all foreign troops from that country."

Voting against the resolution with the Soviet Union, which has veto power as one of the five permanent council members, was East Germany.

Before the vote, Soviet Ambassador Oleg Troyanovsky accused supporters of the resolution of using it as a pretext "to carry out subversive action against the Afghan government."

Troyanovsky repeated assertions of "limited Soviet military involvement in Afghanistan" in response to "repeated appeals for help to repel armed intervention" from countries he did not name.

Afghanistan's "request for aid," he said, was carried out "in accordance with an existing treaty" between the two countries for mutual defense and therefore was a "bilateral matter" of no concern to the Security Council.

Troyanovsky maintained that the overthrow and subsequent execution of Afghan president Hafizullah Amin, which the United States has charged was engineered by the Soviets, was "carried out by genuinely revolutionary elements" within Afghanistan's ruling pro-Moscow party.

Afghanistan's new foreign minister, Shah Mohammed Dost, addressing the council in English, said his government does not deny "the fact of the arrival in Afghanistan of some Soviet Army contingents" who came to "repel foreign armed attacks."

Echoing Troyanovsky, Dost maintained that "the Soviet armed forces were not involved in events of Dec. 27" when the Amin government was overthrown and Babrak Karmal installed as president.

Dost said the Soviet troops, estimated by Washington at more than 50,000, "will not stay one day longer than the source of the present fears" of "foreign intervention" -- presumably from the West -- "continues to exist."

Both Dost and Troyanovsky repeated Soviet allegations that the United States, by maintaining what the Soviet ambassador called "thousands" of military installations throughout the world, was more of an aggressor than the Soviet Union.

The international "uproar" over Afghanistan, Troyanovsky said, was "initiated primarily by the United States," which is "making use of it as a pretext to justify the policy of certain circles in the West in an attempt to take the world back to the Cold War."

Examples of this policy, he said, were U.S. "procrastination in ratifying SALT II" and its decision to deploy new nuclear missiles in Western Europe.

The Soviet position was supported by statements by Mongolia and Laos in today's session. Speaking in favor of the resolution were council members Zambia, Bangladesh, Niger, Tunisia and France, which disputes the Soviet "chronology" of the arrival of its troops and overthrow of Amin. West Germany, Panama, Chile, Canada and Yugoslavia also spoke in favor of the resolution.

Sources said the expected General Assembly session will most likely be requested under the provisions of what is called the "Uniting for Peace" resolution. The 1950 resolution allows issues vetoed in the Security Council to be brought before the assembly within 24 hours.

The United States wants the vote for an assembly session to be called by a Third World member of the council, continuing its efforts to have the developing countries lead the charge against the Soviet Union.Sources said the Philippines, as an Asian council member, is expected to propose the vote but by this evening had not yet received firm instructions from Manila.

The United States, however, seems certain both that the vote will come and that the matter will be passed on to the General Assembly.

"You can expect active discussion on this point," McHenry said "within the next 12 to 24 hours."

A General Assembly session on the Afghan issue would have to be an emergency meeting since the 1979 assembly was adjourned this morning with the election of Mexico to the 15th Security Council seat.

The deadlock over the Security Council seat had been as much between Washington, which supported Colombia, and Moscow, which backed Cuba, as between the contenders themselves.Although Cuba consistently pulled a plurality through 149 General Assembly ballots, it had been unable to garner the necessity two-thirds majority to take the empty seat.

Last Friday, in a dramatic shift in voting patterns, Colombia for the first time received a plurality, which diplomats attributed to general disapproval of the Soviet invasion of Afghanistan, transferred to Cuba as a Soviet surrogate.

In the face of what one informed Latin American diplomat called "an erosion" of support, Cuba withdrew its candidacy, on the condition that Colombia do the same.