U.S. Defense Secretary Harold Brown today held unprecedented talks in Belgrade, marking what officials described as a modest expansion in military cooperation between the United States and Yugoslavia.

According to U.S. officials, one result of the talks is likely to be an increase in American arms sales to Yugoslavia which have been almost minimal since 1961, when Yugoslavia refused to extend a U.S.-Yugoslav military cooperation agreement.

Yugoslavia, which broke away from the Soviet bloc in 1948, has again become heavily dependent on the Soviet Union for procurement of sophisticated arms.

Yugoslavia is understood to be keen to diversify its supplies of arms - even though Yugoslav officials stress that 80 per cent of the Country's arms requirements are produced at home.

Brown's visit to Yugoslavia is the first by a U.S. defense secretary to a Communist state. During his talks with Yugoslav officials, including Defense Minister, Nikola Ljubicic, Brown again stressed the importance that the United States attaches to Yugoslavia's continued independence and territorial integrity.

U.S. officials said Brown made clear that any aggression against Yugoslavia would be regarded in Washington as "a very serious matter indeed, carrying ver strong and very negative implications about European security and peace."

Yugoslav military leaders have drawn up an extensive shopping list of possible arms purchases containing such sophisticated equipment such as wire-guided anti-tank missiles, early warning radar systems, anti-aircraft weapons, and modern communication gear.

Such arms would be used by the country's regular armed forces to delay an invasion, giving time for territorial units to mobilize a guerrilla-type resistance in the mountains.

In public Yugoslav leaders emphasive that an attack could come from any direction, but their military strategy is clearly based on the assumption that it is more likely to come from the East. In an interview published two years ago, Yugoslav generals talked about the need to "destroy at least 2,000 enemy tanks," in the event of an invasion.

Given Yugoslavia's predominantly mountainous terrain, any such tank invasion could only be mounted from Hungary or Romania over the flat northeastern Danubian plain.

While the Soviet Union has sold modest quantities of mobile surface-to-air missiles to Yugoslavia, military strategists believe that it would be more difficult for Soviet planes to avoid an American defense system than a Soviet one.

A previous arms deal fell through two years ago, ostensibly because Yugoslav leaders objected to publicity in American newspapers linking the move to a shift in Yugoslav foreign policy.

Asked about press reports that Yugoslavia recently exported American M-47 tanks to Ethiopia in violation of arms transfer agreements, U.S. officials said this problem had been resolved to the satisfaction of the United States. Yugoslavia is understood to have given assurances that the incident will not happen again.

The United States was Yugoslavia's main supplier of arms during the 1950s, receiving $750 million in U.S. military aid and $1 billion in purchases on favorable credit terms. In recent years, U.S. arms sals to Yugoslavia have been valued at less than $1 million a year, involving spare parts.

Brown, who was accompanied by senior Pentagon officials, also has a meeting with Yugoslav Vice President Stevan Doronjski.