Satellite imagery reveals an escalation in activity at North Korea’s Punggye-ri atomic site in possible preparation for a third nuclear test, experts say. However, there is no clear indication from the photos when the test may occur.

­The images from a US-based website highlight a chain of mining carts on piles of excavated soil. This coincides with South Korean intelligence reports at the beginning of April claiming that the North was digging a new tunnel at the Punggye-ri for the purposes of a covert nuclear test.The US-Korea Institute at John Hopkins School of Advanced International Studies analyzed the photos taken between March 8 and April 18 by a private satellite operator. The most recent images suggest that 8,000 cubic meters of rubble have been excavated at Punggye-ri. North Korea carried out two atomic bomb tests at the site in 2006 and 2009.“While it's very clear from looking at these photos that the North has stepped up preparations for a nuclear test over the past few months, it's unclear exactly when the blast will occur,” Joel Wit, editor of the institute's website said to the Associated Press.He added that upon close examination there has been “a lot of activity at the site” where “you can clearly see vehicles” and objects being moved around.The international community has rounded on North Korea recently for what it views as an escalation in aggressive activities.Experts say the country possesses enough plutonium for several “simple bombs”, but as of yet lacks the capability to construct bombs small enough to mount on a missile.Washington has voiced concerns that North Korea seeks to develop an intercontinental missile capable of carrying a nuclear warhead.UN Secretary General Ban Ki-moon warned Pyongyang not to make any "further provocative actions or conduct any nuclear test." He branded the country’s failed attempt to test launch a missile on April 12 as a “threat to international peace and security.”In an embarrassing turn of events for North Korea, the rocket disintegrated shortly after take-off. Pyongyang claimed the launch was to put a satellite in orbit, while the international community suspected this was a front to test a missile capable of carrying a nuclear warhead.As retaliation for the launch, the US cancelled a deal to deliver food aid to North Korea. White House spokesperson Jay Carney warned the country on Wednesday to "refrain from engaging in any more hostile or provocative actions. They do nothing to advance the cause of peace on the Korean peninsula or in Northeast Asia." He added that the government’s policies do little to help the Korean people, “many of whom are starving.”Image from latimesblogs.latimes.com