The United States is forging a new and stronger relationship with Argentina but is concerned that Argentina's developing nuclear capability will lead to nuclear proliferation in South America.
Strategically, a key reason behind improved relations between the US and Argentina is mutual concern about communist expansion, particularly in Central America and Bolivia.
Politically, the Reagan administration appears pleased with the Argentine military regime's policy of liberalization. On balance the country's human-rights record is regarded as having improved enough to lift an arms embargo set by the 1977 Kennedy-Humphrey amendment, according to reliable sources.
Final decision on the embargo appears to be delayed because of the murder in Chile of a leading Chilean trade union official. The US does not want to upset the military balance in the southern cone by opening the possibility of arms sales to one country and not the other, sources say. But the US worries less about conventional military balance than about a potential nuclear arsenal in South America.
''The US does not recognize the distinction between peaceful devices and atomic weapons. Let there be no doubt about that,'' Thomas Enders, US assistant secretary of state for inter-American affairs, told reporters in Buenos Aires recently. But so far there has been little US pressure on Argentina to refrain from going ahead with nuclear development.
Increasing speculation that Argentina might be close to developing a nuclear device by diverting some of the resources used in its nuclear power program were fueled recently when the head of the country's atomic energy commission told the the Atomic Industrial Forum that Argentina intended to push ahead with its plans for nuclear ''autonomy'' by producing plutonium.
The commission head, Adm. Castro Madero, then rejected Mr. Enders' warning and reaffirmed Argentina's ''right'' to make an atomic bomb for ''peaceful ends'' as a contribution to his country's development.
Mr. Enders, the highest-ranking US official to visit Argentina since a bloodless coup put Gen. Leopoldo Galtieri in power last December, says the two countries agree that ''collective action is necessary'' on Central American issues.
Argentina has stood by the Reagan administration's position to support El Salvador's military-civilian junta and the Salvadoran election in March before the Organization of American States and the United Nations. Argentine military advisers are reported to be operating in El Salvador, and the country will send observers to the elections. It has also offered military and economic assistance to El Salvador.
According to military sources in Buenos Aires, details of this assistance were discussed during the recent visit here of Col. Rafael Flores Lima, El Salvador's chief of staff.
The US and Argentina are also in tune over Bolivia. Both countries are concerned about the ramifications of any possible political and economic collapse there, and both are considering ways of supporting the military regime.
Argentina has provided several credits to help its debt-ridden neighbor with balance-of-payments problems, and it maintains a strong military presence in La Paz to advise the local armed forces.
US officials say that Argentina is close to having a capability to build a bomb. International efforts to limit the countries with nuclear capability have proved inadequate to prevent the country from moving ahead. Argentina has refused to ratify the nonproliferation treaties.
So far it is generally believed that Argentina has not developed a nuclear bomb, although it has the capacity to do so.
Concern over Argentina's nuclear ambitions dates back to the country's legendary Gen. Juan Peron. On taking power after World War II, Peron made military self-sufficiency a top Argentine priority.