President Carter yesterday announced new goals for reducing overseas sales of conventional weapons during the coming year but said future U.S. cutbacks will depend on co-operation from other nations selling arms.

The announcement in a written statement is the clearest expression yet of developing shift from self-imposed and unilateral arms sales restraints to a policy that would depend on international agreement.

Previous administrations considered formal U.S. commitments to restrain weapons sales to be impractical because there was no sign that other nations would go along. But Carter initially took the view that the United States, as the world's largest arms merchant should put its own house in order and only later seek cooperative arrangements.

So far the Carter administration's attempts to convince other supplier nations have been unsuccessful European countries, some of which are highly dependent on arms exports in foreign trade, have declined to join the U.S. effort ahd have shown little interest in a multilateral program of control over arms sales along U.S. lines.

The U.S.-Soviet talks on conventional arms transfers, which showed promise several months ago, are scheduled to resume Tuesday in Mexico City without much prospect of early programs.

Informed administration officials said this round of talks will be shorter than other recent U.S.-Soviet meetings on the same topic. The officials are no longer hopeful, as they were this summer, that the coming round may achieve "concrete results" toward superpower limitations on weapons sales.

In his statement yesterday, Carter noted that he had previously told the United Nations that "we could not go very far alone" toward restraining weapons transfers. He added that international cooperation remains essential to curbing arms sales, and said the United States is "making a maximum effort" to achieve it.

His statement announced a fiscal year 1979 ceiling of $8,434 billion on U.S. weapons sales to all but closely allied countries, a slight reduction in dollar terms from this year's $8,551 ceiling and a reduction of 8 percent when adjusted for inflation.

"My decision on U.S. arms transfer levels for fiscal year 1980 will depend on the degree of cooperation we receive in the coming year from other nations, particularly in the area of specific achievements and evidence of concrete progress of arms transfer restraint." Carter said. This is the first time that he has made future U.S. ceilings conditional on what others do.

One of the major items in the Mexico City talks with the U.S.-Soviet joint efforts to curb arms sales to Latin America, where the United States has long observed some self-imposed limitations on the supply of advanced weaponry and where the Soviet Union has relatively few arms clients.

The Soviet Union's close political, military and arms-supply relationship with Cuba - Moscow's major ally in the area - are a complicating factor in the drive for regional curbs. The recent Soviet supply to Cuba of Mig 23 warplanes, which has been questioned by the United States in diplomatic conversations, has cast a new cloud of controversy over the regional restraint discussion.

In addition to Latin America questions, about a half-dozen other matters are reported to be on the agenda for discussion with the Soviets in Mexico City. U.S. officials would not explain why they are less optimistic than before about making tangible progress in this round of talks, but the decline in optimism was notable.

Carter's arms sale ceiling does not apply to U.S. weapons transfers to NATO countries, Japan, Australia or New Zealand. Critics have pointed out that weapons sales actually have grown under the Carter ceiling, from $11.4 billion in fiscal year 1977 to $13.8 billion this year, if the arms to all countries are counted.

White House officials would not estimate the total expected U.S. sales in fiscal year 1979 if the countries exempted Carter's ceiling are included.

Carter called restraint in the supply of weapons "an important objective of this administration and the Congress." His statement argued, as in the past, that curbing the flow of weapons is in the interest of all nations because regional arms races threaten stability and divert national resources from other needs.