PYONGYANG, North Korea (AP) -- A "special envoy" for North Korean leader Kim Jong Un left Pyongyang on Wednesday for China, the North's only major political and economic benefactor. State media released few details, but the trip comes at a rocky time in ties between the allies.

In a short dispatch, the North's official Korean Central News Agency said Kim's envoy was Choe Ryong Hae, the North Korean military's top political officer tasked with supervising the army. Choe was one of a handful of new vice marshals North Korea announced last year.

The trip comes as a new leadership in China has shown frustration with North Korea and a greater willingness to work with Washington to harry Pyongyang over its nuclear ambitions. China is Pyongyang's economic and diplomatic lifeline, providing nearly all of its fuel and most of its trade.

Still, Beijing's primary goal is stability, and it has sought to temper diplomatic pressure on the North's nuclear program so as not to create a crisis that could damage the Chinese economy or push refugees into China.

South Korean Unification Ministry spokeswoman Park Soo-jin said it's too early to provide an assessment on Choe's trip to China.

Kim Jong Un hasn't visited Beijing since he took power from his father, Kim Jong Il, who visited China in August 2011 just months before he died that December.

Strains have increasingly appeared in relations between Beijing and Pyongyang over North Korea's nuclear efforts, which included a February nuclear test, Pyongyang's third. That test was followed by U.N. sanctions and a weekslong period of high tensions, as North Korea threatened nuclear strikes on Washington and Seoul. Those tensions have eased some, and there have been tentative signs of an interest in diplomacy.

In another sign of Beijing's displeasure with its ally, China's state-run Bank of China Ltd. said earlier this month that it had notified the Foreign Trade Bank of North Korea that its accounts were closed and all financial transactions suspended. Washington welcomed the move.

On Tuesday, a Chinese fisherman said gunmen wearing North Korean military uniforms released his boat after holding its crew for two weeks, beating up the captain and stealing the vessel's fuel.