The United States will attempt to evacuate 1,700 Americans from Iran this weekend with the help of Iran's revolutionary government, which yesterday rescued U.S. Ambassador William H. Sullivan and his staff from an assault by leftist guerrilla forces in Tehran.

The government appointed by Ayatollah Ruhollah Khomeini has assured the Carter administration it will attempt to get Tehran's Mehrabad Airport open by Saturday to permit U.S. military aircraft to resume massevacuation flights, U.S. officials said.

The White House's Special Coordination Committee, composed of Cabinet-level officials, reportedly met in emergency session last night to consider new evacuation measures.

The closing of Iran's airports two weeks ago, as the struggle between Khomeini's forces and the deposed shah's supporters came to a head, trapped an estimated 7,000 Americans in Iran. Administration officials told reporters yesterday they hope to airlift 5,000 Americans out of Iran in a matter of days, using routine measures.

Despite the evident shock of watching one of its largest and most important embassy compounds fall into the hands of guerrillas, the administration drew political encouragement from yesterday's incident, which State Department spokesman Hodding Carter suggested would help in building a normal relationship between the two governments.

"The activities of the provisional government are very encouraging to us," Carter told reporters. He said the administration was "most grateful" for the Iranian regime's "expressions and the action taken."

Throughout three unusual briefings at the State Department, the first of which began shortly after dawn, Carter stressed that his information on the siege and its lifting by Khomeini's forces was fragmentary. The department had no direct communications with the embassy, he said.

At the third briefing at 5 p.m. yesterday, Carter said that 18 of the Americans taken prisoner by the guerrillas and removed from the embassy compound had not returned by the end of the day. He said their whereabouts were known "and they seem to be physically okay."

The spokesman refused to discuss evacuation plans or to tell reporters if the embassy takeover had changed the administration's assessment of how forcefully it should urge U.S. civilians to leave Iran. He said he wanted to avoid statements that could "be fed back into Iran by one voice or another" to incite anti-American sentiment.

The 1,700 Americans who are expected to jam aboard the first military aircraft that arrive at the Tehran airport have been waiting for flights for about five days, Carter said. More Americans may want to leave immediately in view of yesterday's embassy attack, he added.

The number of Americans in Iran, which at one time was estimated at 45,000, had shrunk to about 10,000 by last month, when the administration ordered the evacuation of all dependents and nonessential military and diplomatic personnel, and strongly advised U.S. civilians to leave.

State and Defense Department officials told a meeting of congressional staff aides that the government of Prime Minister Medhi Bazargan had given its assurances to the administration to get the airport in Tehran opened immediately after the religious holidays today and tomorrow as Khomeini consolidates his control.

Pentagon officials said the U.S. Air Force's Military Airlift Command has assembled extra aircraft and crews at bases in West Germany and Greece to pick up Americans in Tehran if the State Department requests a quick evacuation.

"We are in touch with the Iranian government in an effort to facilitate the possible evacuation of Americans there," Hodding Carter said.

Last night's White House deliberations on evacuation and embassy security were held in the absence of President Carter, National Security Affairs Adviser Zbigniew Brzezinski and Secretary of State Cyrus R. Vance, who are on a state visit to Mexico.

Also absent were Defense Secretary Harold Brown, who is on a Middle Eastern tour, and Gen. David Jones, chairman of the Joint Chiefs of Staff, who is in the Philippines.

Early yesterday, before leaving for Mexico, Vance was notified at home by telephone of the seizure of the embassy. He arrived at the State Department shortly before 1 a.m. and awakened the president with the news two hours later, spokesmen said.

Vance and Carter briefly considered not making the trip, but decided to leave at 8 a.m. after they heard that Sullivan and the embassy staff had been rescued by what one official termed armed "irregulars" representing Khomeini.

By nightfall in Tehran, security of the embassy was completely in the hands of the irregulars, 40 of whom were stationed inside the compound, Hodding Carter told reporters. He said the Iranian embassy in Washington had relayed to the State Department explicit assurances from Foreign Minister Karim Sanjabi, speaking for Khomeini and Bazargan, that the embassy will be fully protected.

Hodding Carter's praise for the cooperation offered by Khomeini's forces was in sharp contrast to the critical remarks about the Moslem leader during Khomeini's determined and eventually successful campaign to bring down the shah, one of President Carter's favorite friends abroad.

Stressing that the department did not "have a clear picture" of the attacking group, Carter characterized the invaders as "leftist Marxists." He said this description was based on conversations embassy staff members had with the guerrillas during the takeover, but he offered no details.

He asserted that the administration "has good reason to believe" that "adequate security measures had been taken" to protect sensitive material before the guerrillas established full control over the embassy. His phrase was intended to convey that secret documents had been destroyed, despite news agency accounts from Tehran quoting U.S. officials as saying that some sensitive material had been left unprotected during the siege.