North Korea's Kim Jong Un is most likely to conduct the country's sixth nuclear test in the middle of next month, a diplomatic source told South Korea's Yonhap News Agency on Monday. The news comes as Pyongyang continues its threats against the United States, South Korea and other neighbors.

Increased activities at North Korea's Punggye-ri test site had earlier given rise to speculation that the next nuclear test could be any time soon.

"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."

The source also reportedly said: "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 U.S. and China (in early April) or Kim Il-sung's birthday (on April 15). It could be done around April 25 when it celebrates the anniversary of the Korean People's Army."

Read: North Korea Will Launch Rockets 'At Any Time' Kim Jong Un Pleases, Officials Say

On Sunday, North Korea warned the U.S. and South Korea that it will conduct a "strike without warning" in retaliation to the Washington-Seoul joint military drills.

“They should be mindful that the [North Korean Army] will deal deadly blows without prior warning any time as long as the... 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,” North Korean news agency KCNA said Sunday, citing a statement from the General Staff of the Korean People's Army (KPA).

“The KPA will not remain a passive onlooker to hordes of robbers trying to hurt our people with daggers,” it added.

As tensions escalate in East Asia, a South Korean military source said the North most likely has finished preparing for another nuclear test and that it is capable of conducting it "within hours of Kim Jong Un's order."

North Korea conducted its fourth and fifth nuclear tests in January and September last year, drawing strong condemnation from the international community, forcing them to impose tough sanctions against the reclusive country.

Earlier this month, North Korea fired four ballistic missiles into the sea off Japan's northwest coast on March 12 amid escalating tensions with South Korea and Japan. The missiles, which were launched from the Dongchang-ri long-range missile site in North Pyongan Province, flew about 620 miles before hitting the Sea of Japan.