Tong Zhao, a fellow at the Carnegie-Tsinghua Center for Global Policy in Beijing, said the visit was unlikely to be an attempt to strong arm Pyongyang over its nuclear program.

"It has become a sort of routine for the Chinese Communist Party to send a special envoy to its fellow communist and socialist countries to brief them on China's Party Congress," he said.

"China did so after the conclusion of the 2007 and 2012 party congresses, so this is not really unusual."

Zhao, however, said it was "very likely" the North Korea nuclear program would come up as part of their discussions.

Repairing ties

John Delury, associate professor at the Yonsei University Graduate School of International Studies in Seoul, said China would take the temperature of North Korea's ruling elite and reopen communication channels apparently shut as ties had soured.

"Everyone's looking for a breakthrough on the impasse over the nuclear missile program and obviously that's part of the mix but ... this looks more like the main focus is on improving the Xi, Kim channel somewhat and seeing what can be done," he said.

"Even just at a basic level of getting a sense of where Kim Jong Un is at and what are the possibilities," said Delury. "We're in the very delicate stage and you need to get a read, you need to have lots of conversations. The Chinese channel is one of them."

Zhao said there was unlikely to be any major breakthrough during the Pyongyang visit on reining in North Korea's nuclear program: "It could well be routine exchange of their long standing positions."

Delury said it would be key to monitor how Song is received by the North Koreans and who he meets:

"The way that the North Koreans report the visit, who he meets, and the tone of the reporting will tell us something about whether it was positive and some progress made, or whether it was a surly exchange," he said.