The Argentine government announced today that it is unilaterally suspending payment on some $1.4 billion in external debt falling due in March and April.

Although today's announcement did not specify the duration of the suspension, a Central Bank spokesman said that the government soon will release a plan for rescheduling the payments.

This is the first time that Argentina had announced it would not make external payments without setting a timetable for settling the debts.

Argentina, with a foreign debt of $38.7 billion, is believed to be the fourth largest debtor in the developing world behind Mexico, Brazil and South Korea. The government is negotiating with banks to refinance $12 billion in principal payments falling due this year, and in January won approval for $2.1 billion in loans from the International Monetary Fund.

In a related development, United Press International reported yesterday that Peru asked its commercial bank creditors for $880 million in new loans and presented a three-point plan to roll over roughly $2 billion of its $11.5 billion in foreign debt that falls due through early 1984.

Carlos Rodriguez-Pastor, Peru's minister of economy, finance and commerce led a delegation of officials who met in New York with some 120 of the country's 250 international commercial bank lenders to present its proposal.

[Peru, which has an external debt of some $11 billion, entered into a three-year agreement with the IMF last April.]

Argentina's Central Bank officials and financial analysts described today's action as part of a plan to concentrate resources on the payment of $2.7 billion in overdue interest charges from l982 by mid-year.

Nevertheless, financial analysts here said that the unilateral action might provoke a sharp reaction from foreign banks. Bank officials disputed Argentina's earlier unilateral rescheduling of foreign-exchange contracts and still are discussing possible adjustments in that plan with the government, these analysts said.

The obligations in question were in "swap" agreements under which banks helped Argentina meet balance-of-payments deficits in l98l and l982 by providing sums of dollars in exchange for pesos at a fixed exchange rate. In return, Argentina promised to return the dollars for the pesos at the same exchange rate when the agreements expired.

[From Basel, Switzerland, Reuter news agency reported yesterday that leading Western central banks agreed to make Yugoslavia a $500 million bridging loan to help it meet repayments on its foreign debt totaling about $19 billion, according to central bank sources.]

And in Washington, Martin S. Feldstein, chairman of President Reagan's Council of Economic Advisers, said yesterday that the United States would come to Mexico's financial assistance if the price of oil dropped sharply, the Chicago Tribune reported.

If the average oil price fell by $10 a barrel, Mexico's annual export earnings would drop by more than $5 billion, and it would not be able to pay off its debts, Feldstein said.

["I believe that, in the event of a sharp decline in the price of oil, the United States government would recognize the special nature of our relation with Mexico," he said.]