According to Global Fellow Chen Jian, the Sino-DPRK treaty does not obligate China to support North Korea if Pyongyang were to attack the US.

The “Treaty of Friendship, Cooperation and Mutual Assistance between the People’s Republic and the Democratic People’s Republic of Korea” of 1961 was drafted in a hurry. 

Although there had been general discussions between the Chinese and North Koreans concerning a potential treaty of alliance prior to mid-1961, no substantial negotiations or progress had been made.

Only after North Korean Foreign Minister Pak Seong-cheol informed Chinese Ambassador Qiao Xiaoguang that Kim Il Sung would visit the Soviet Union on June 29, 1961, with the explicit goal of signing a treaty of mutual assistance, did the Chinese rush to make actual progress. Between June 26 and June 30, Beijing and Pyongyang agreed that the PRC and DPRK would also sign a treaty of friendship, cooperation, and mutual assistance.

After visiting Moscow, Kim Il Sung revised his itinerary to include a stop at Beijing and on July 11, 1961, he and Chinese Premier Zhou Enlai signed the PRC-DPRK treaty. The text of the treaty mimicked the language of the DPRK-USSR treaty almost exactly, except for one major difference: while the DPRK-USSR treaty carried a ten-year term, renewable with one-year advance notice, the PRC-DPRK treaty did not include an end date. In theory, it is still in effect today. 

The text of the PRC-DPRK treaty clearly states that only when one side is attacked by another country or group of countries, the other side must come to its assistance. Therefore, if North Korea were to attack the United States, China is not bound by treaty obligation to support it.

On two occasions during the Cold War Beijing acted in relation to the treaty. After the Pueblo incident in January 1968, the PRC issued a strong statement expressing "firm support" for the DPRK, despite the fact the PRC-DPRK relationship was then at a low point. It warned: “If the US imperialists attempt to use war bluff to scare the Korean people, they should not forget the lessons of the Korean War. If they dare to risk new military aggression, they would be inviting sufferings, and would be even more severely punished.” 

On the second occasion, however, the Chinese were not so staunch in their support. On the eve of communist victory in Indochina during April 1975, Kim Il Sung traveled to Beijing to gain the PRC's backing for his plan to use a "revolutionary war" to unify the Korean peninsula. He announced after arriving in Beijing that in such a war “We will only lose the military demarcation line and will gain the country's unification.” However, Mao Zedong showed no interest in Kim’s plan, and Deng Xiaoping essentially vetoed it. 

This month’s statement from Beijing on its treaty obligations to Pyongyang treads new ground. Never before has China stated so explicitly that it would not come to North Korea’s aid in a war against the United States.