Western Europe's defense ministers today assured America that their countries would shoulder more of the burden of defending Europe from Soviet attack to permit the United States to bolster its military presence in the Persian Gulf and Indian Ocean.

While formal European pledges come Thursday, U.S. officials said they were pleased with the "very positive" response today from Europe to a number of stepped-up defense efforts requested by America in the wake of the Soviet invasion of Afghanistan.

Part of the reason for Europeans' willingness is the modest nature of the measures asked of them so far. But Europe, too, seems to have concluded that the Afghanistan invasion represents a threat to the Western alliance and that the Soviet action requires a strengthening of the West's military forces.

This theme is expected to be underlined Thursday when foreign ministers join defense ministers meeting here at the first formal session of the NATO Defense Council since the Soviet invasion last December.Its message seems intended as a strong signal to the Soviet Union and a reminder to the Western alliance itself to spur the lagging defense modernization programs approved by NATO two years ago.

The Soviet invasion has presented the alliance with a double military dilemma: first, how to respond firmly to the Soviets and, second, how to compensate in Europe for America's new defense commitments to the Persian Gulf and Indian Ocean.

President Carter's pledge to protect the West's oil sources in the Middle East has meant that some U.S. forces, originally earmarked to reinforce allied defenses in Europe in the event of a Soviet attack, might now be available if needed. Already, the United States has shifted one aircraft carrier and several destroyers formerly assigned to Europe from the Mediterranean to the Indian Ocean, according to U.S. defense officials. At the same time, U.S. officials say there are no plans to withdraw U.S. troops now stationed in Europe.

To take up the slack, the United States has pressed Europe to accelerate previously planned NATO improvements. The speedup comes in two phases.

The first phase requires that the allies commit to a number of "quickfix" measures to bolster NATO in Europe. These are generally modest steps to be formally pledged by Europe tomorrow. They include:

Increases in European ammunition reserves.

Improvements in chemical warfare defense measures.

Greater military assistance to the countries along the alliance's southern flank, especially Turkey.

The second phase addresses a number of longer term items:

Improvements in electronic warfare defenses.

Increased prepositioning of war equipment in Europe to lessen the reinforcement effort that would be required in the event a real conflict flares.

Alterations in commercial widebodied jets flown by European airlines so that the planes can be easily converted for military use to carry U.S. troops from America to Europe.

More training and heightened readiness for European reserve units.

Further buildups in European ammunition reserves.

This second phase is expected to be studied, but U.S. defense officials say they would like to see final approval at the next ministerial meeting in December. Together, these programs add up to what American officials regard as just the opening rounds in a gradual shifting of Europe's defense onto allies' shoulders.

But the outlines of this evolution are still vague and are likely to be frustrated by a lack of European money and initiative.

Specifics as to what is being requested of each European country vary. But all have been asked to reaffirm their 1978 pledge to increase defense spending by 3 percent annually.

Both Belgium and Denmark, however, probably will not meet this target and there is some concern in NATO councils that other countries may also fall short this year because of high inflation rates and other economic and political pressures at home.

Moreover, although the United States is only seeking to speed up programs already approved, and while the European response at the highest levels has been generally favorable, there is grumbling detectable at lower levels.

In one remark critical of U.S. defense officials for "coming in with new priorities every year," a knowledgeable official of a powerful European country said in a recent interview, "One can disturb a good program with too much wind and waves."

To underline Europe's existing contribution to the NATO military effort -- it amounted to about 40 percent of the total in 1979 -- European defense ministers yesterday approved a special public relations budget to finance trips by North American journalists to Europe.