The latest round of East-West talks on military forces reductions in Central Europe ended yesterday with officials reporting substantial progress for the first time in the five-year-old negotiations.

U.S. officials here said that recent moves by NATO and the Warsaw Pact have established a "framework" that combines common elements" of the two sides and thus allows for "more serious negotiations."

Officials cautioned, however, that "significant differences" concerning strength estimates will face the two sides when they resume the Mutual and Balanced Force Reduction (MBFR) talks in Vienna in September.

Optimistic statements here and in Vienna at the end of the 15th round of talks yesterday followed a Warsaw Pact initiative last month that appeared to close a long-existing gap.

The new Soviet bloc initiative accepts the Western concept of a common ceiling of 700,000 men for each side. The Soviets suggested cutbacks that would eventually reduce the overall Warsaw Pact forces by 105,000 and NATO by 91,000. The proposal included initial cutbacks of 30,000 Soviet troops and 14,000 U.S. troops in Central Europe. A second-stage trim would involve 25,000 Soviet and 10,000 U.S. troops.

All along, the United States and its allies have insisted on "asymmetrical cuts" that would involve greater reductions of Eastern forces. This argument was justified by the superior numbers of the Warsaw Pact's men and tanks, by geographic advantages and by the proximity of the Soviet borders to Central Europe.

For years, the Soviets and their allies have rejected the idea of asymmetry as well as the common ceiling for the combined forces of both alliances.

Moscow's acceptance of asymmetrical cuts that would lead to a common ceiling is now complicated by the discrepancy between Western and Soviet data on the size of Warsaw pact troops in Central Europe.

The Warsaw Pact insists that the combines strength of its forces in the area is 805,000. NATO estimates set the combined strength at more than 950,000. The NATO estimate is almost 150,000 higher - the equivalent of at least 10 divisions.

The central European theater involves the forces of the Soviet Union, East Germany, Poland and Czechoslovakia on the Eastern side, and the United States, West Germany, Britain, Canada, the Netherlands, Belgium and Luxembourg on the Western.

By Western accounts, the Communist side would have to trim 250,000 men to reach the 700,000-man ceiling and not the 105,000 envisaged in the Soviet proposal.

Since the military dispositions in Europe have been fairly stable for many years, the Western side believes that its intelligence estimates are accurate.

A spokesman for NATO said yesterday that "although we have had a useful discussion on data this round, there is still no clarification of the discrepancy between Western and Eastern figures on Eastern military manpower in the area."

Although encouraged by the Soviet acceptance of the principle of common ceilings, U.S. officials said that no agreement is possible before the discrepancy on data is resolved.

One apparently disturbing provision of the Soviet plan would permit the "outside powers" - which means the Soviet Union and the United States - to actually increase their military manpower in Central Europe as their allies cut back.

Some Western specialists suggested that this provision would allow the Soviets to arrange for the forces of their allies to fall behind and have the Soviet Union make up the difference up to the overall ceiling.