South Korea’s Yonhap News Agency quoted an unnamed Beijing-based functionary as saying, "It concerns me that the North is doing excavation work at two tunnels (of the site)," the diplomatic source based in Beijing told Yonhap. "Lots of analysis and evaluation will be needed to determine whether both of them, when completed, will be used to conduct nuclear tests simultaneously or at a short interval and what nuclear materials will be used."
He also speculated on when the test mike take place, suggesting, "There are chances that the North will conduct an additional nuclear test, though I cannot say for sure that it will come around the summit between the US and China (early April) or Kim Il-sung's birthday (April 15) … it could be done around April 25 when it celebrates the anniversary of the Korean People's Army."
Over the last year and a half the DPRK has conducted over 25 missile launches and two atomic explosions as part of its effort to attach a nuclear warhead on a long-range missile.
The rumblings come as Pyongyang threatens a preemptive strike against South Korea and the US during their joint military drills, releasing a statement saying the two countries "should be mindful that the [North Korean Army] will deal deadly blows without prior warning any time as long as the… as long as troops of the US and South Korean puppet forces involved in the 'special operation' and 'preemptive attack' targeting the DPRK remain deployed in and around South Korea…The KPA will not remain a passive onlooker to hordes of robbers trying to hurt our people with daggers."
Kim Jong Un threatened a week ago to reduce the US "to ashes" if their military conducts a pre-emptive strike against Pyongyang.
North Korea conducted four ballistic missile tests earlier in March from the Dongchang-ri long-range missile site in North Pyongan Province, with the projectiles travelling over 600 miles before falling into the Sea of Japan.
Prime Minister Shinzo Abe called the launches "extremely dangerous action," before Parliament, and US Press Secretary Sean Spicer said the North was engaging in "provocative behavior."
Washington has been critical of the communist nation lately, though no cogent policy has yet been drafted.
On March 17 President Donald Trump wrote on Twitter that the DPRK had been "behaving very badly," while his Secretary of State Rex Tillerson heavily implied that the US would engage military action against Pyongyang if provocation continued.