Washington — Fresh tremors, sparked by war in the Middle East, are shaking the international financial system, already strained by the huge debt burdens of Argentina, Poland, and other countries of Eastern Europe.
''If the Lebanon fighting were to invoke retaliation by oil-producing states, '' says US Treasury Secretary Donald T. Regan, ''it would be very alarming.''
In the past few days, Mr. Regan told reporters at breakfast, the price of crude oil on spot markets has risen to about $33, $3 above the price prevailing during the early spring.
Other signs that the ''markets are very nervous,'' he said, include a strengthening of the dollar -- considered a safer haven than other moneys by many investors -- and a weakening of the Japanese yen.
More than 75 percent of Japan's total oil supplies, indispensable to the smooth running of its economy, comes from the Middle East. The comparable figure for the United States is about 9 percent.
Western Europe also depends more heavily than the US on shipments of Mideast and North African oil -- a major reason that Europeans want to substitute imports of Soviet natural gas for some of their Arab oil.
Vulnerability of Japan and Europe to any cutoff or reduction in the flow of Arab oil tends to drive the value of their currencies down and the dollar up in times of crisis.
In Paris, the dollar appears to be moving toward a record high against the French franc. The unresolved Falklands crisis, together with the Middle East war , buoys the dollar against the pound sterling.
''Much depends,'' said Mr. Regan, ''on whether the (Mideast) war spreads or is contained. An enormous effort is under way to bring this conflict to a halt.''
He referred particularly to President Reagan's tentative plan to send Secretary of State Alexander M. Haig Jr. to Israel and the journey of Saudi Arabian Foreign Minister Saud al-Faisal to Bonn to confer with Mr. Reagan.
Most experts agree that Arab oil exporters, plus Iran -- all scrambling to keep or improve their world market shares -- do not want to invoke the ''oil weapon.''
Arab governments, however, would come under heavy pressure to punish countries that appear to favor Israel -- first of all the United States -- if the war spreads.
Another conflict - the war between Iraq and Iran at the head of the Persian Gulf - appears to be entering a new phase. Iraq's leaders have declared what they term a unilateral cease-fire, ostensibly to divert their forces to the struggle against Israel.
Earlier, the Iranians -- whose forces appeared to be the verge of military victory against Iraq -- had rejected a cease-fire move offered by Baghdad.
If peace does develop in the Gulf, both Iran and Iraq would strive to expand their oil exports, which were sharply reduced during their drawn-out conflict.
A quick end to the fighting in Lebanon might calm the surface waters of international markets, but deep currents of uncertainty would remain.
''Argentina,'' notes William R. Cline, senior fellow at the Institute for International Economics, ''has about $10 billion of short-term debt coming due this year, plus another $3 billion to $4 billion of long-term debt.''
cl11 So far, says Mr. Cline, Argentina appears to be meeting its debt obligations, partly by paring down imports and maintaining exports -including grain to the Soviet Union -- at substantial levels.
Of Argentina's total foreign debt, estimated from $32 billion to $35 billion, US banks hold a bit more than $9 billion -- roughly four times as much as in 1975. American petroleum, chemical, and other firms also have about $2.5 billion worth of direct investment in Argentina.
Although Argentina's overall debt is greater than Poland's $26 billion, Buenos Aires can earn large amounts of money through exports -- a capability Poland currently lacks.
But the assumption that Argentina can pay its debts through exports presupposes that the end of the Falklands war will leave the political structure of Argentina relatively intact. Prolonged political turmoil could darken the debt-payment picture.
Poland, meanwhile, appears to be unable to meet its interest and principal payments due this year without new loans from a consortium of 500 US and other Western banks.
Poland owes American banks about $2.1 billion, according to the State Department, including $800 million in US government-guaranteed loans for the purchase of American farm products.
''If the (war in Lebanon) does not threaten oil supplies,'' says Mr. Cline, ''it will put less of a strain on the international financial system than Argentina and Poland, because (the Middle East) does not involve debt.''