The leaders of China and North Korea vowed on Sunday to enhance bilateral cooperation on the 75th anniversary of diplomatic relations between the two countries, Pyongyang's state news agency KCNA reported.

"I attach great importance to the development of the China-DPRK relations," China's President Xi Jinping said in his message to North Korean leader Kim Jong Un, using the initials for North Korea's official name, the Democratic People's Republic of Korea.

Xi said China was ready to continue to "write a new chapter" of relations through strengthened communications and cooperation.

China is by far North Korea’s largest trading partner. The two share a history going back to Beijing’s support of Pyongyang during the 1950-1953 Korean War that resulted in the formation of North and South Korea.

But China reacted guardedly in June as Kim deepened ties with Russian President Vladimir Putin. This year, North Korea and Russia signed a strategic treaty that includes mutual defense elements.

In his message to Xi, Kim pledged to "steadily strive to consolidate and develop the friendly and cooperative relations between the DPRK and China as required by the new era."

North Korea is expected to revise its constitution to abandon its long-held goal of peaceful unification with the South and to redefine the inter-Korean border, a move which experts say could escalate tensions on the peninsula. North Korea will convene a parliamentary session to discuss the amendment on Monday.

In January, Kim called for changing the constitution to ensure that South Korea is seen as the "primary foe" and that North Korea should plan for "occupying, subjugating and reclaiming" South Korea in the event of a war.

There are concerns that North Korea would suggest that it would pursue unification by force, including nuclear weapons, under the revised constitution, said Lim Eul-chul, a professor of North Korea studies at South Korea's Kyungnam University.

The redefining of the inter-Korean territory also could be a "new flashpoint in disputes between the two Koreas," he said.

On Friday, Kim said his country would not hesitate to use all available offensive forces including nuclear weapons if the enemy attempted to use force to encroach on its sovereignty.