North Korea’s nuclear test has presented Beijing with a stark test of foreign policy, less than four months into Xi Jinping’s leadership, in a move that could further escalate diplomatic tensions across the region.

Beijing on Tuesday said it “firmly opposed” Pyongyang’s nuclear tests and had summoned the North Korean ambassador in Beijing to the foreign ministry for a démarche. Chinese foreign minister Yang Jiechi urged North Korea to “put a stop to words and deeds that exacerbate the situation, and immediately return to the right path of dialogue and consultation.”

China is by far North Korea’s biggest formal ally, but the relationship has started to fray after a series of diplomatic snubs by Pyongyang since Kim Jong-eun came to power a year ago. The deteriorating ties led to China’s decision to support a fresh round of UN Security Council sanctions against North Korea last month.