Pyongyang said earlier this year that it did not rule out a fourth nuclear test, to counter what it described as US provocations in Asia.
Satellite imagery also confirmed continued activity at North Korea's main nuclear test site of Punggye-ri that was consistent with preparations for an atomic detonation.
Construction work meanwhile continues at the light water reactor at Yongbyon, which was initially shut down in July 2007 but which Pyongyang said last year it would restart to bolster its atomic arsenal.
"All these activities clearly show that the DPRK is further developing its nuclear capabilities," the EU said in its statement, which was backed by Turkey, Norway, Ukraine and other European countries.
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"This can only further increase our concerns," it added, urging Pyongyang to allow IAEA inspectors into the country again after North Korea kicked them out in 2009.
North Korea has carried out three nuclear tests since 2006. The last, and most powerful, was in February 2013.