BEIJING, AUG. 21 -- China and the Soviet Union, following a new round of bilateral talks, indicated today that they were making progress toward resolving a longstanding dispute over their heavily armed 1,500-mile border.

According to an unusually positive joint statement, the latest discussions have proved "beneficial to deepening mutual understanding and promoting a settlement of the boundary question." The two sides agreed to establish working groups of experts to discuss the eastern sector of the border, where it runs along the Amur and Ussuri rivers.

China's Vice Foreign Minister Qian Qichen said, "As regards principles, the opinions of the two sides were completely in accord . . . but we are still a long way from success." East European diplomats here also cautioned that no breakthrough had occurred and that many difficult obstacles remain to be overcome before a settlement is achieved.

The major obstacle in the eastern sector is the disposition of a disputed, Soviet-held island opposite the Soviet city of Khabarovsk. The Soviets maintain that the island, which houses military installations, is vital to the security of the city.

A first round of talks was held in Moscow in February of this year after a break of nine years without negotiation over the disputed borders.

The agreement to resume talks this year was hastened by a concession from Soviet leader Mikhail Gorbachev. In July of last year, Gorbachev said that the Soviet Union was willing to accept China's position that the Amur river border passes along the river's middle line, or main ship channel. The Soviets had previously insisted that the border be placed along the Chinese river bank.

The Chinese side, for its part, appears to have dropped a demand that the Soviet Union should recognize that border treaties between Russia and China agreed upon in the days of the czars were unequal.

The statement issued by the two sides today said they both "stand for a reasonable settlement" in the eastern sector on the basis of the existing treaties and in accordance with the principle that the main navigational channel should be the boundary in the case of a navigable river.

Soviet Deputy Foreign Minister Igor Rogachev told reporters before leaving Beijing today that "we believe the talks have been fruitful.

"We have grounds to feel very satisfied," the Soviet negotiator said.