TOKYO, Feb. 20--Communist China today accused Secretary of State Dean Rusk of threatening the Vietnamese and Chinese people with a "big war" and warned that China is prepared to take up the challenge.

In a broadcast from Peking, the New China News Agency cited Rusk's refusal to state any limit on American military involvement in South Vietnam at Friday's hearings before the Senate Foreign Relations Committee as proof that "U.S. imperialism is determined to seize South Vietnam by force."

But the broadcast stopped short of saying that Chinese troops would be sent into the Vienamese war. It said only that that the Chinese will fight if the war is extended to the China mainland. According to some China observers in Asia, Poking now has a real fear that the United States may eventually carry the air war beyond North Vietnam into China.

Kennedy Proposal Scored

WASHINGTON, Feb. 20--Administration officials took a cold view yesterday of a suggestion by Sen. Robert F. Kennedy (D-N.Y.) that a coalition government in Saigon might be necessary to end the Vietnam war.

McGeorge Bundy, special assistant to President Johnson for security affairs, and Undersecretary of State George Ball disagreed with him.

Kennedy told a news conference Saturday that the alternative to killing off all the Viet Cong is to bring its political arm, the National Liberation Front, to the bargaining table. He said to "admit them to a share of power and responsibility is at the heart of a negotiated settlement."

Bundy said on the NBC program "Meet the Press" that the administration does not take the view "that admitting the Communists to a share of the power and responsibility would be a useful or helpful step... one that would really lead to peace."

He added that what is needed is peace and an end to subversion and terror so there can be reasonable expression of the political feeling of the people in Vietnam.