WASHINGTON -- North Korea has accelerated excavation at a site used for underground nuclear test explosions, though a test doesn't appear imminent, a U.S. research institute said Thursday.
The findings, based on satellite photographs, were released as Secretary of State John Kerry and his South Korean counterpart warned the North against any possible aggression.
The U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies said that the North likely started work last May on a new tunnel at the northeastern test site at Punggye-ri.
ducted its three previous nuclear explosions, the latest in February 2013. The institute estimates that the pile of earth excavated from it has doubled since the start of the year.
The findings are based on commercial satellite photographs, the latest taken Feb. 3. The analysis was provided to The Associated Press ahead of publication on the institute's website, 38 North.
It adds to doubts over North Korea's intentions.
Pyongyang has dialed down the threatening rhetoric it issued following the international condemnation of its last nuclear. The authoritarian government of young leader Kim Jong Un has since said it's willing to hold nuclear talks without preconditions and is now pushing for quick improvement of ties with rival South Korea.
Yet its nuclear and missile development appears to continue unabated.
Two weeks ago, Director of National Intelligence James Clapper said that the U.S. believes that North Korea has expanded the size of its uranium enrichment facility at its main nuclear complex and restarted a reactor that was used for plutonium production before it was shut down in 2007 -- reversing a concession previously made in disarmament-for-aid negotiations with the U.S., China, Japan, Russia and South Korea. Clapper also said the North has taken initial steps toward fielding a road-mobile intercontinental ballistic missile, although it remains untested.
"We have yet to see evidence that North Korea is prepared to meet its obligations," Kerry said in Seoul Thursday. "Let me be clear; the United States will not accept North Korea as a nuclear-armed state. We will not accept talks for the sake of talks, and the DPRK must show that it will live up to its commitments," he said, referring to the North by the initials for its official name, the Democratic People's Republic of Korea.
The U.S. is "ready and able to deter North Korean aggression," he told a news conference. "It is time for the North to choose the path of peace and refrain from provocations or using excuses to avoid the responsibility that they bear."
South Korean Foreign Minister Yun Byung-se said that both Seoul and Washington "stand fully prepared against any potential situation given the mixed signals from North Korea, even as it continues a charm offensive."
On Wednesday, senior North and South Korean officials held their highest-level talks in years and are set to meet again Friday to finalize arrangements for their first reunions since 2010 of families divided by the 1950-53 Korean War. The emotional reunions are set to start next week, although a stumbling block remains.
The North is demanding a delay in the South's springtime military exercises with the U.S., which the North calls a rehearsal for invasion. The allies say the drills are defensive in nature. The North wants them pushed back until the reunions are complete on Feb. 25.
The U.S. and South Korea say the issues should not be linked. The North has cited what it calls America's "hostile" policy as a reason to rescind an invitation to a U.S. diplomat to visit the country to discuss the case of jailed American missionary Kenneth Bae.
The North also has an eye on resumption of lucrative cooperative ventures with the South.
North Korea will also be an important agenda item on the next leg of Kerry's three-nation Asian trip.
He travels Friday to China, the North's only major ally and its principal trading partner. The U.S. views increased cooperation with Beijing as the key to pressing Pyongyang to hold meaningful negotiations that can lead to its eventual nuclear disarmament.
"There is more that China can do," Kerry said Thursday.
Washington and Beijing both share the goal of denuclearization on the Korean Peninsula. The U.S. is increasingly concerned about the North's progress toward a nuclear-tipped missile that could target America, and China doesn't want the North Korea to trigger an atomic arms race in East Asia or see instability spill across its southern border.
But Washington and Beijing are at odds over how to start the wheels of diplomacy turning. Beijing wants the U.S. to start talking with Pyongyang, while the U.S. wants nations involved in the stalled six-nation nuclear talks to forge a common stance, demanding that North Korea first take concrete steps to show it is committed to denuclearization.
The new imagery from Punggye-ri is the latest sign that North Korea is moving in the other direction. 38 North estimates that the amount of heaped earth suggests a new tunnel around 1,000 meters long has been dug into the mountainside. Mining carts can be seen on the tracks to the earth pile, it says.
38 North says that North Korea probably has two other tunnels already complete at the site. According to Yonhap news agency, this week South Korea's Defense Minister Kim Kwan-jin told lawmakers in Seoul that North Korea appears ready to conduct its fourth nuclear test, although they have not detected signs that a test is imminent.
The entrances of the two other tunnels are in shadow and not visible in the latest overhead images -- underscoring the limitations on what can be gleaned about the North's plans, particularly when the crucial activity takes place underground, out of view.
"North Korea is clearly keeping all of its options open for the future," said Joel Wit, a former State Department official and editor of 38 North. "There is no evidence to suggest that Pyongyang is preparing for another nuclear test, but if a decision were made tomorrow, it could conduct a blast probably by late March or April."
Associated Press writer Matthew Lee in Seoul, South Korea, contributed to this report.