The Soviet Union today denounced Secretary of State Alexander M. Haig Jr.'s offer to sell U.S. arms to China, Moscow's bitter foe, as new proof of Chinese-American military expansionism that "poses a serious threat to Southeast Asia."

In the first extended Soviet reaction to Haig's Peking visit, which ended last night, the official Tass news agency said the bilateral talks "demonstrated that Peking has gone even further in its conspiracy with U.S. imperialism."

Aside from "strengthening the two countries' anti-Soviet policies," Tass said, the Haig mission "gives grounds for legitimate concern not only to the Asian countries neighboring China, against which Chinese expansionism is directed, but also to other parts of the world."

The statement also indicated that the talks had dampened Moscow's latent hopes that the Reagan administration's strong backing for Taiwan would lead to serious difficulties with China. "The Tawian question featured prominently in the talks," Tass reported. "Haig said he had explained to the Chinese the U.S. intention to continue maintaining relations with Tawian at the unofficial level and this was met with understanding by the Chinese side. In other words, Peking has again consented to the creation of a "two Chinas" situation by the American side."

Tass quoted Haig as saying, "The Chinese think highly of measures by the U.S. to built up U.S. military potential and strengthen contacts with its allies in military blocs. Washington for its part understands this commonality as encouragement of the hegemonistic and aggressive aspirations of Peking above all in South and Southeast Asia."