The United States and the Soviet Union may be on the threshold of a cooperative new era in arms control, but they have yet to make any breakthrough on resolving regional conflicts where they are competing hard for influence.

While administration officials have warmly welcomed a new flexibility and frankness in regular meetings between U.S. and Soviet experts on regional disputes over the past three years, the two sides have yet to move "beyond diologue to cooperation or a mutual understanding on regional conflicts to eliminate problems at the sources," said a State Department official who has been intimately involved in the discussions.

Nor have they led to any agreement on mutual restraints in U.S.-Soviet Third World rivalries. Instead, the two superpowers view each other with deep suspicion and mistrust, each citing examples of the other's bad faith in trying to settle such conflicts as those in the Persian Gulf, Afghanistan and Angola.

"Regional conflicts have been left behind and we sense they are not going so well as other issues in U.S.-Soviet relations," said the State Department official.

"I don't think we have come to grips with regulating our rivalry in the Third World," said Raymond L. Garthoff, a specialist in Soviet affairs at the Brookings Institution. "Each side finds it useful to have a dialogue but it's on the margins."

The regional conflicts most likely to be on the agenda at next week's summit between President Reagan and Soviet leader Mikhail Gorbachev are Afghanistan and the Iran-Iraq war.

The administration is pressing Gorbachev to set a "date certain" for withdrawal of an estimated 115,000 Soviet troops from Afghanistan. Moscow has hinted strongly it is ready to do this as part of a package deal that includes a U.S. cutoff of military aid to the Afghan resistance and help in settling up a coalition government.

U.S. officials say they are ready to be "helpful" on the first point but say "it's their war" and "up to them," as one put it, to engage the rebels on a political settlement.

Cooperation there could set the tone for other cooperation. "Quite frankly, it's a very important test of the ability of the United States and Soviet Union to cooperate on regional conflicts," said Deputy Assistant Secretary of State Edward P. Djerejian.

In the Iran-Iraq war, administration officials want to know if Gorbachev will back a U.N. resolution imposing an arms embargo on Iran for refusing to accept a cease-fire and negotiated settlement with Iraq.

A U.S.-Soviet effort at the United Nations last summer to pressure Iran into negotiation has all but collapsed. U.S. officials say they are "seriously disappointed" by Soviet refusal to begin discussing an embargo resolution.

Hopeful after the superpowers worked together to gain unanimous passage of a U.N. Security Council resolution last July demanding an end to the Iran-Iraq war, U.S. officials now accuse the Soviets of shielding Iran and abetting it by a "stalling game" at the United Nations.

The failure to cooperate worries administration officials who remember how the Soviet invasion of Afghanistan in December 1979, following Soviet gains in Ethiopia, Angola and Cambodia, brought the short-lived era of detente to a bitter close and ended any possibility of Senate ratification of the SALT II Treaty.

"It is a truism of detente in the 1970s that regional issues were neglected," a State Department official remarked.

The Soviet actions were seen as violating the Basic Principles Agreement signed by President Richard M. Nixon and Soviet leader Leonid Brezhnev at their 1972 Moscow summit. Among other things, the agreement said that any effort by one side to obtain "unilateral advantage at the expense of the other" in Third World conflicts would be "inconsistent" with the principles of detente.

Secretary of State George P. Shultz said yesterday that "whatever is emerging now" in U.S.-Soviet relations is "quite different from what was so in the 1970s."

The Soviets seem mindful of the impact their Afghanistan incursion has had on relations with the United States, and now they are suggesting readiness to resolve the issue that sealed the SALT II Treaty's fate.

But U.S. officials are wary that Soviet hints of withdrawal from Afghanistan may be only part of a public relations campaign to sell the new intermediate-range nuclear forces (INF) treaty to the Senate.

"A cynical view would see it as a way of getting through the INF ratification process here," said a State Department official.

The skepticism reflects general administration doubts about whether substantive change is behind Moscow's new policy of stressing political rather than military solutions to regional conflicts and "national reconciliation" between warring parties to resolve them.

"When we take a hard look at the Soviet positions as they relate to the Persian Gulf, the Arab-Israeli peace process and to a lesser extent Afghanistan, you see this gap between words and deeds and statements of intent and the actual situation on the ground," Djerejian said.

Conservative analysts agree on this. "I don't yet detect any fundamental, genuine longlasting changes in actual Soviet international behavior," said W. Bruce Weinrod, a Soviet analyst at the Heritage Foundation, the conservative think-tank here. "But there have been important changes in the packaging of their policies to give the image of greater flexibility."

U.S. officials note that the Soviets have even copied administration terms, such as "national reconciliation," when pressing their allies in Cambodia, Afghanistan and Nicaragua to engage in talks with the opposition.

But in war-ravaged Ethiopia and Angola, the Soviet Union and Cuba continue to provide billions of dollars in arms to help local Marxist regimes impose a military, rather than a political, solution on long-festering political conflicts.

Meanwhile, U.S. officials believe the Soviets seek "national reconciliation" only on terms that will leave their local communist, or pro-Moscow, allies in power while dissolving the opposition.

They point to the fact that Cambodia's communist leaders, reportedly under Soviet pressure, have just opened talks in Paris with opposition leader Prince Sihanouk, offering him a top government post if he will come home. But there is no evidence they will give him a dominant role in any coalition government.

Soviet officials seem to harbor as much doubt about the administration's real intentions. They note that Reagan has made a "doctrine" out of providing hundreds of millions of dollars in military aid to anti-communist forces around the world to overthrow Moscow's Third World allies.

Soviet officials say they doubt the administration is willing to make real concessions, or deal with Moscow on an equal footing, to resolve regional conflicts. They cite as examples U.S. efforts to shut them out of the Persian Gulf and continuing administration ambivalence about a substantive Soviet role at any Middle East peace conference.

One Soviet official who has been intimately involved in U.S.-Soviet discussions on a possible Mideast peace conference said here recently the two sides are "still far away" from any agreement. He complained that the United States shows no willingness to accept the notion Moscow, like Washington, has legitimate interests in the Middle East.

Soviet officials also cite as an example of what they call the administration's bad faith its decision early this year to send hundreds of Stinger antiaircraft missiles to the Afghan resistance, just as Moscow was launching its "national reconciliation" policy in search for a way out of Afghanistan.

"It seems to me that this testifes to the fact that you want to inhibit the process of withdrawal of our troops from there," remarked Evgeny Primakov, a chief Middle East adviser to Gorbachev. He was speaking on the recent ABC "Capital to Capital" television program.

U.S. officials say there is no administration intent "to bleed the Soviet Union dry" in Afghanistan, as Djerejian put it, but that Soviet behavior "on the ground" there raises doubts about their real intentions.

They note that the Soviets have so far supported the Afghan communist leader Najibullah in his efforts to set up a coalition government dominated by the Afghan Communist Party and to change the constitution, making himself all-powerful politically.

AFGHANISTAN: Soviet Union sent an invasion force to install a pro-Soviet Marxist government in December 1979. U.S. aid to the Afghan resistance now tops $600 million annually and includes hundreds of Stinger missiles.

CAMBODIA: Vietnamese forces crossed the border and occupied the capital, Phnom Penh, in January 1979. Soviets currently spend $1 billion to $2 billion annually to support the Hun Sen government and a 140,000-man Vietnamese occupation force. U.S. supports noncommunist resistance groups.

NICARAGUA: United States organized and supports a 10,000 to 15,000-man rebel force, known as contras, to fight the Soviet/Cuban-backed Sandinista regime. Soviets now provide $1 billion in aid to Sandinistas. Cuba has sent hundreds of advisers.

ETHIOPIA: Soviet Union replaced the United States as main foreign power in 1977. Soviets have provided $4 billion in arms to Marxist government, while the U.S. has backed several opposition groups with little success.

PERSIAN GULF: Iran and Iraq have been fighting since September 1980, with about one million casualties to date. U.S. has tilted toward Iraq. Soviet Union is a main arms supplier to Iraq but recently sought closer ties with Iran. U.N. Security Council passed a mandatory cease-fire resolution in July but Iran refused to accept it.