China said that it has been making unremitting efforts on Pyongyang's nuclear issue, in response to a tweet from US President Donald Trump saying Beijing had tried but failed to help resolve it.

Foreign Ministry spokesman Geng Shuang said at a daily news briefing that China was not the "focus and the crux" of the issue on the Korean Peninsula.

"In order to resolve the Korean Peninsula nuclear issue, China has been making unremitting efforts and we have been playing an important and constructive role," Geng said.

"To sum up, our contributions are recognized by all and our efforts are indispensable."

Trump wrote on Twitter on Tuesday that while he "greatly appreciated the efforts of President Xi Jinping and China" to help with Pyongyang, "it has not worked out".

"At least I know China tried!" he wrote.

Since assuming office in January, Trump has demanded that Beijing put pressure on Pyongyang to curb its nuclear and missile programs.

Geng said China has tried its best with the DPRK on the issue because it is a responsible member of the international community, and "not because of any pressure from other countries".

Top Chinese and United States diplomats and defense chiefs were meeting in Washington for talks on Wednesday, and Pyongyang was to get "top billing", said Susan Thornton, the senior US diplomat for East Asia, according to AFP.

Tao Wenzhao, a senior US studies researcher at the Chinese Academy of Social Sciences, said China made its best efforts on the Korean Peninsula issue, but China does not have the key to solving the issue, as Foreign Minister Wang Yi has said.

"The pivot in solving the issue lies with Pyongyang's improved relations with Washington and Seoul."

Zhou Jin contributed to this story.