President Carter offered little hope last night that the twin crises in Iran and Afghanistan will be resolved soon as he began an effort to solidify public support behind his handling of the situations.

At a White House briefing for about 80 members of Congress, the president said he did not expect any of the retaliatory actions he ordered last week against the Soviet Union to cause the Soviets to withdraw their troops from Afghanistan.

But Carter said it was important for the United States to act swiftly to let the Soviets know that actions such as the invasion of Afghanistan will produce consequences.

"We hope that we have laid down a marker and let them know that they will indeed suffer, now and in the future, for this unwarranted invasion," he said.

The president added that he considered the invasion "the greatest threat to peace since the Second World War."

On Iran, Carter said the United States is no closer to a solution than it has been since the Nov. 4 seizing of the American hostages and embassy in Tehran.

According to participants at the meeting, he portrayed Iran's revolutionary leader, Ayatollah Ruhollah Khomeini, as believing he is "an instrument of God" and therefore above the possibility of making errors in judgments or decisions.

The White House meeting was the beginning of a concerted effort to maintain support for administration policy in Iran and explain the retaliatory measures Carter ordered last week against the Soviet Union following its invasion of Afghanistan. Today, some 40 private citizens, described by officials as active and interested in foreign policy, have been invited to a similar meeting at the White House, with other meetings planned for later.

The president was joined last night by Secretary of State Cyrus R. Vance and officials of the Treasury and Agriculture departments.

According to participants, in response to questions Carter said that China has not asked for U.S. military aid and that he considered it inadvisable "to enter into a military relationship with China at this time."

Playing "the China card" against the Soviets, the president was quoted as saying, would only increase Soviet "paranoia" over China and stiffen Soviet resistance to demands for a withdrawal from Afghanistan.

Much of the meeting was taken up with questions about the partial grain embargo Carter ordered against the Soviets and about steps the administration has taken to protect American farmers from economic losses.

The president put the price tag of the farmers' aid at $2.8 billion, including at least $2 billion this fiscal year.

The response of U.S. allies to requests for cooperation in the effort to deny certain products to the Soviets is "adequate," Carter said. Yesterday French President Valery Giscard d'Estaing told him that Western European nations would not make additional agricultural products -- particularly barely, of which they have a surplus -- available to the Soviets.

Rep. Michael D. Barnes (D-Md.) said Carter received an "extremely positive" response from the legislators last night. Dissenting, Sen. Roger W. Jepsen (R-Iowa) said he remained unimpressed with the plan to aid U.S. farmers and called the limited grain embargo "the worst event for Iowa's economy since the Depression."