Tokyo — The Soviet Union offered China an agreement guaranteeing nonuse of nuclear weapons, a senior Japanese official said. Mikhail Kapitsa, Soviet deputy foreign minister, referred to the offer when he visited Tokyo last week for talks, said Yoshiya Kato, Japan's director general of the Foreign Ministry's European Affairs Bureau. But Mr. Kato said he didn't know whether any agreement had been reached on the Soviet proposal. Last week, the Soviet minister offered Japan a similar agreement. But Japan rejected it, saying a nuclear-armed nation had a duty not to attack nonnuclear countries.