The EU said Wednesday it was "deeply concerned" about the threat of a new nuclear test by North Korea, which has stepped up activity at one of its main test sites.

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.

"The EU is deeply concerned about reports that the DPRK (Democratic People's Republic of Korea) may be preparing the fourth nuclear test," EU states said in a joint statement to a meeting of the UN watchdog IAEA's board of governors in Vienna.

"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.

"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.

"The EU demands that DPRK abandons all its existing nuclear and ballistic missile programmes, including its uranium enrichment programme, in a complete, verifiable and irreversible manner, and refrains from any further provocative actions and statements," it said.

North Korea has carried out three nuclear tests since 2006. The last, and most powerful, was in February 2013.