A sudden halt in Argentina's lucrative grain trade with Iran has upset President Raul Alfonsin's budget plans at a crucial stage of debt refinancing talks with overseas creditors.

Argentina suspended all grain shipments to Iran in mid-February, reportedly because Iran was delaying payments.Since then, local bankers have asked themselves how the Argentine government will finance its planned budget deficit in the first quarter of this year.

Curbing the budget shortfall is a key issue in talks aimed at persuading the International Monetary Fund to disburse a long-delayed standby loan, which in turn would free $500 million in funds from Argentina's bank creditors.

Iran has paid Argentina hard currency for grain supplies in recent years, and had ordered 1.5 million metric tons of hard wheat from Argentina for delivery during February and March. About 400,000 metric tons of wheat were loaded at Argentine ports before the dispute broke out. Apparently, those cargoes have not yet been delivered.

Grain trade and shipping analysts in Buenos Aires say several grain- carrying vessels are tied up in the Straits of Hormuz awaiting orders to proceed to Iranian ports.

Reports say five other vessels chartered by Iran have been waiting for three weeks to load at Bahia Blanca, Argentina's leading Atlantic coast grain export terminal.

According to banking analysts, the scheduled first quarter wheat shipments would have been worth some $150 million (about 945 million australs at the current exchange rate of 6.30 australs to the dollar), and this would have gone a long way to meet the budget shortfall during the first three months of the year.

Iran was Argentina's third-largest market for grain and oilseed exports in 1987, after the Soviet Union and Holland, and was set to move up into the first position during the first three months of this year.

So far, Argentine officials have played down the decision to halt grain shipments, blaming reported payment problems on bureaucratic delays or technical difficulties. But a previously unannounced visit to Buenos Aires by Iranian Foreign Minister Mohammed Javad Larijani this week is seen as a sign that the dispute has assumed serious proportions.

Little has been said about what happened when Mr. Larijani met with Mr. Alfonsin on Monday, although a senior Argentine official said that grain trade was not discussed explicitly.

The absence of official information has not halted speculation about why Iran fell behind with payments, or perhaps stopped them altogether.

Local traders link the dispute to Iranian demands that Argentina's grain and other exports be exchanged for goods on a countertrade basis.

Iranian officials recently told Argentine meat exporters that Tehran was willing to buy an average 2,000 metric tons of fresh beef this year if Iran was allowed to meet all payments in crude oil, one usually well-informed source said.

Similar conditions are said to have been attached to a promise to private grain traders in Argentina that Iran was interested in taking at least 500,000 metric tons of corn, and perhaps twice that much, during the second quarter of this year, the source added.

At around $88 a metric ton, half a million metric tons of corn would be worth some $44 million to the Argentine treasury in the April-June period.

Now, bankers say, doubt hovers over the budget deficit in the second quarter as well.

Reaction from the Argentine shippers to Iranian insistence on countertrade deals was highly negative, sources said, amid growing irritation at Iran's failure to pay.

Market analysts say it remains to be seen whether the halt in payments reflects a cash crunch in Iran, or a ploy by Tehran to force a major structural change in bilateral trade policy with Argentina.