Seoul - Recent satellite imagery suggests North Korea is excavating a new tunnel at its main nuclear test site, although there are no indications that a test in imminent, a US think tank said Thursday.
The tunnel is in a new area of the Punggye-ri site, separate from three other tunnels that the North has excavated or used for actual tests in the post, the US-Korea Institute at Johns Hopkins University said.
"While there are no indications that a nuclear test in imminent, the new tunnel adds to North Korea's ability to conduct additional detonations over the coming years if it chooses to do so," the institute said.
The latest satellite photos were taken in October and November, but the institute said a review on imagery over the past year showed construction on the new tunnel may have begun as early as April.
The North has conducted three nuclear tests in 2006, 2009 and 2013 - all at the Punggye-ri site in the country's northeast.
The institute's imagery analysis would appear to confirm South Korean reports in late October that a fourth tunnel was being excavated.
In September, North Korea confirmed the restart of a nuclear reactor seen as its main source of weapons-grade plutonium.
North Korea mothballed the Yongbyon reactor in 2007 under a six-nation aid-for-disarmament accord, but began renovating it after its last nuclear test in 2013.
When fully operational, the reactor is capable of producing around six kilos (13 pounds) of plutonium a year - enough for one nuclear bomb, experts say.