Two of the world's largest borrowers -- have reached tentative agreement with the International Monetary Fund on emergency loans in exchange for belt-tightening measures, financial sources here said yesterday.

Meanwhile, a third major borrower, Brazil, has obtained $600 million in emergency cash from six U.S. banks to help cover its balance-of-payments deficit and tide it over until it can reach agreement with the IMF, banking sources in New York said yesterday. Brazil has not officially asked the IMF for a loan.

The IMF has provisionally approved a $3.8 billion three-year loan to Mexico, whose financial crisis this summer sent tremors through the world banking system, and a $1.6 billion loan over 15 months to Argentina, Washington sources said yesterday. An additional credit of about $500 million is expected to be granted to Argentina under a special IMF lending facility, sources said. Mexico originally hoped for extra money under this facility, which compensates for sudden losses in export earnings.

The two IMF accords -- which have yet to be approved formally by the international agency's board of directors -- commit Argentina and Mexico to austerity measures aimed at reducing their dependence on foreign borrowing, sources said. However, both agreements assume private banks will resume lending to the two nations, with some new money being made available as well as existing credits being continued, sources said.

Mexican officials are expected to meet their private creditors in New York on Friday, sources said. A group of private banks overseeing the negotiations between commercial banks and Mexico met yesterday.

Mexico has been haggling for months with the IMF. It now has agreed to sharp cutbacks in public spending aimed at reducing its yawning budget deficit, the sources said yesterday. Although the accord has been reached with the outgoing government of President Jose Lopez Portillo, representatives of President-elect Miguel de la Madrid were present at all the negotiations with the IMF and have agreed to the deal, sources indicated.

The new government takes office on Dec. 1 and is due to present its budget on Dec. 15. It is likely to include cuts in a wide range of subsidies, such as those on tortillas, electricity consumption and gasoline, sources said.

Formal IMF approval of the Mexican and Argentinian loans normally would take about six weeks, while papers are prepared and circulated. This process may be speeded up for the Mexican loan, monetary sources said.

The loan to Argentina runs for only 15 months because elections are due at the end of that time.

The IMF accord should pave the way for agreement between Mexico and its many private bank creditors on a stretch-out of loan repayments and an infusion of new bank credit, sources said. Mexico has an estimated $85 billion of foreign debts in total, and was forced to go to the IMF for emergency cash this summer when private banks stopped lending it the new money on which it depends to pay for imports and service its existing debts.

The crisis led to a wave of anxiety among bankers about the huge amounts of money they had at risk in developing countries. Bankers' growing reluctance to lend out still more money to Latin America after the Mexican crisis put strains on other nations such as Brazil that are dependent on new loans to meet their import bills and service debts.

Brazil owes foreigners about $80 billion to $85 billion, roughly the same amount as Mexico.

The six U.S. banks that have agreed to lend $100 million each to Brazil are Bank of America, Citibank, Chase Manhattan, Manufacturers Hanover, Morgan Guaranty Trust, and Chemical Bank.

Banking sources said that they expect Brazil will announce new austerity measures, similar to the type the International Monetary Fund would be expected to demand, next week. Because Brazil will hold national elections next Monday, sources said any official announcement of either the new loan agreements or any austerity measures probably will be delayed until after the election.