Chinese President Xi Jinping has sent a congratulatory letter to North Korean leader Kim Jong-un, who has been given the new title of the chairman of the Workers' Party of Korea during a key party congress, the North's state media said Tuesday.

The North's official Korean Central News Agency (KCNA) said that Xi sent the letter to Kim on Monday.

The key party congress, the first in 36 years, opened Friday and ended Monday.

In the letter, Xi said he offers "fervent" congratulations to Kim on behalf of the Central Committee of the Communist Party of China, according to the KCNA. He also said that the Chinese party and government "highly cherish" the relationship between Beijing and Pyongyang.

Portraying the two countries' traditional neighborly ties as a "shared invaluable asset," Xi added that China will work together with the North to contribute to regional peace, stability and development.

The sending of the letter came amid several years of diplomatic estrangement between the two countries due to Pyongyang's continued provocations, including a fourth nuclear test in January and a series of ballistic missile launches early this year.

China has recently vowed to fully enforce the U.N. Security Council sanctions that were imposed on the North in early March in response to its latest nuclear test and long-range rocket launch in February. The move had further dimmed the prospect of a thaw in the Beijing-Pyongyang relations.

But amid stepped-up efforts to strengthen security cooperation among South Korea, the U.S., and Japan, China might have felt the need to improve ties with its ally, the North, said Hwang Jae-ho, an international politics professor at the Hankuk University of Foreign Studies.

"China might have recognized the strategic value of the North as its rival the U.S., South Korea and Japan are working closely together to build stronger defense relations," he said.

Analysts had also predicted that Pyongyang would seek a way to mend ties with Beijing, its only major ally and patron, to ease its deepening international isolation and shore up its debilitated economy. 

Since Xi took power in 2013, he has yet to hold any bilateral summit with the reclusive North Korean leader, which has underscored the diplomatic tension between the two neighbors.

The North's dogged pursuit of nuclear arms has been cited as the biggest stumbling block to the bilateral relationship as China has called for a nuclear-free Korean Peninsula and stressed the importance of stability along its border regions. (Yonhap)