SEOUL (AFP) - Recent satellite imagery shows no sign of North Korea preparing an imminent nuclear test, despite a rise in military tensions on the Korean Peninsula, a United States think-tank said on Wednesday.

North Korea has ramped up the volume on its bellicose rhetoric in recent weeks in response to the launch of annual South Korean-United States joint military exercises.

Pyongyang's condemnation of the drills has included veiled threats of a nuclear test, but the US-Korea Institute at Johns Hopkins University said satellite pictures offered no indication that any detonation was in the works.

While the North's Punggye-ri nuclear site is "at a sufficient state of readiness to move forward with a test if ordered to do so... there are no signs of preparations", the institute said in an analysis posted on its closely followed 38 North website. "As a result, the North is unlikely to conduct a nuclear or missile test over the next few months," it added.

North Korea is already subject to a raft of United Nations and US sanctions imposed as the result of its three previous nuclear tests, carried out in 2006, 2009 and 2013.

Pyongyang is currently believed to have a stockpile of some 10 to 16 nuclear weapons fashioned from either plutonium or weapons-grade uranium.

A recent report by US researchers, including the US-Korea Institute, warned that North Korea appeared poised to expand its nuclear programme over the next five years and, in a worst case scenario, could possess 100 atomic arms by 2020.