SEOUL (Reuters) - The president of China, which has grown increasingly frustrated over North Korea's pursuit of nuclear weapons, sent a congratulatory message to the North's leader Kim Jong Un on his promotion to chairman of the country's ruling party, North Korean state media said on Tuesday.
The 33-year-old Kim, the third-generation leader of isolated North Korea, added the title of chairman of the Workers Party of Korea (WPK) on Monday, the closing day of the country's first ruling party congress in 36 years.
"I wish the Korean people fresh success in carrying out the cause of socialism under the leadership of the WPK headed by Chairman Kim Jong Un," Chinese President Xi Jinping told Kim, according to North Korea's state KCNA news agency.
"We will make efforts together with the DPRK side to bring happiness to the two countries and their peoples and contribute to peace, stability and development in this region by steadily developing the Sino-DPRK friendship and cooperation," Xi said.
DPRK stands for Democratic People's Republic of Korea, the North's official name.
China, North Korea's chief ally and main trading partner, backed tough United Nations Security Council sanctions against North Korea in March following the North's fourth nuclear test, in January.
During its ruling party congress, North Korea said it would strengthen its nuclear weapons capability, and said it would not use nuclear weapons unless its sovereignty is infringed by others with nuclear arms.
(Reporting by Tony Munroe; Editing by Richard Pullin)