China is North Korea's longtime treaty ally and main economic benefactor, their relationship forged in the bloodshed of the Korean War when Mao Zedong sent in millions of "volunteers" to fight US-led United Nations forces to a standstill.

Mao once described the two countries as being close as "lips and teeth", and while relations have fluctuated over the years, a breakdown in negotiations between North Korea and the United States has spurred both sides to strengthen their alliance.

"The traditional China-DPRK friendship has weathered the trials of the changed international situation for a long time," Xi said in a message to North Korean leader Kim Jong Un, using the initials for the North's official name.

In the message carried by the Korean Central News Agency, Xi noted the current international and regional situations are "changing seriously and in a complicated way", adding: "I will continuously push the friendship and cooperation between the two sides to a higher stage."

The message came just hours before G7 foreign ministers demanded North Korea "refrain" from further nuclear tests and ballistic missile launches, warning of a "robust" response.

"Such actions must be met with a swift, united, and robust international response, including further significant measures to be taken by the UN Security Council (UNSC)," the diplomats said in a statement after talks in Japan.

Pyongyang has carried out a blitz of banned weapons tests this year, including firing several of its most powerful intercontinental ballistic missiles (ICBMs).

Last week, the North hailed its first successful test of a new solid-fuel ICBM, saying it will "radically promote" the country's nuclear counter attack capabilities.