According to this Reuters article:
North Korea has told its key ally, China, that it is prepared to stage one or even two more nuclear tests this year in an effort to force the United States into diplomatic talks, said a source with direct knowledge of the message.
Kim Jong-un throws a bone to allies in China by giving them the heads-up. The question is: how much do the Chinese want to be seen benefiting from their insider status with NK. It probably is not the kind of image Beijing wants to cultivate internationally – in part because uncomfortable parallels between the regimes in Pyongyang and Beijing. Especially since in recent years, the Chinese have dipped their toe in the water of liberalisation here and there.