French Foreign Minister Claude Cheyson warned today that the Reagan administration could provoke "major difficulty" with its European allies, especially France, if it fails to give high priority to improving relations between underdeveloped and industrialized countries.

He said the so-called North-South dialogue is one of the two major agenda items -- along with high U.S. interest rates -- that France wants discussed at the seven nation noncommunist summit conference in Ottawa later this month.

Cheyson told an Anglo-American Press Association luncheon: "We're heading for a serious situation -- the word is not too strong -- if a problem which has top priority for us, for the American is only marginal, secondary and postponable. pThen we would have a major difficulty between us."

A major North-South conference originally scheduled to take place last spring "was postponed at Reagan adminstration request and now has been rescheduled for this fall in Mexico.

Hammering away at by-now standard themes of President Francois Mitterand's Socialist-dominated government, Cheyson said, "No one has properly explained American [economic] policy to us yet."

But despite criticism of American reliance on high interest rates alone to stem inflation, Cheyson reiterated his government't claims to being that the staunchest American ally on East-West matters.

As if to stress that point, Premier Pierre Mauroy and a large group of other Socialist ministers and party officials attended U.S. Ambassador Arthur Hartman's Independence Day reception today.

Cheyson noted that France alone of Western nations had said its relations with Moscow would be affected as long as Soviet troops remained in Afghanistan.

The United States is "lucky" to have a country like France as an ally, he said, adding that France, unlike "certain neighbors" that he did not identify, is untouched by the "virus of neutralism."

So, too, did France agree with American condemnation of the presence of Vietnamese troops in Cambodia, he said.

But he said France was "not convinced" that the Reagan administration's reliance on high interest rates is the best way to fight inflation.

He advocated a mix of intervention on foreign exchange markets and budget restraint, which he said the Reagan administration's increased defense spending and tax reductions are endangering.

Left unchecked, the administration's policy would produce 30 percent interest rates in Europe and further worsen European unemployment, he said.

The effects of American policy on Belgium already are "catastrophic," Cheyson said.

Regarding policy toward the underdeveloped world, Cheyson said, "For certain Americans -- I do not say President Reagan -- the South does not exist."

For them "there are certain countries in the South which are strategically interesting and others that are economically promising," he said, but he added that the U.S. government lacks any overall understanding of the North-South problem.

European nations are more dependent on access to the underdeveloped world than is a continent-size country like the United States, Cheyson said. He noted that foreign trade accounted for 28 percent of France's gross national product, compared to 8 percent for the United States.

Cheyson also returned to a favorite Socialist theme in calling for calm and understanding in Central America.

He said the situation in any country right after a major upheaval "is far more complex than a man in a beard and jungle fatigues."

He said that while he was still a commissioner of the European Economic Community, he had found it "incredible when we were treated with suspicion" by the Reagan administration "for sending $3 million worth of medicine and blood serum" to El Salvador.

He said that the United States had been right when it provided food and medicine to keep Algeria afloat right after its independence in 1962, when Algerians found all things French suspect.