Recent satellite imagery suggests North Korea is unlikely to conduct a fresh nuclear test in coming months, according to a US think-tank.

The Stalinist state recently turned up the volume on its hostile rhetoric and boasted of its nuclear and missile capability, claiming it had successfully tested a submarine-launched ballistic missile last month.

But the US-Korea Institute at Johns Hopkins University said satellite pictures indicate no signs of preparations for its fourth nuclear test being made at this time.

"Recent commercial satellite imagery indicates that North Korea is conducting regular spring construction and maintenance activities at its Punggye-ri nuclear test site," the institute said in a report by analyst Jack Liu on its website 38 North.

"There are no indications of nuclear test preparations at this time. Given the time and effort such preparations require, North Korea is unlikely to conduct another nuclear test until at least fall 2015 at the earliest," it said.

The North has conducted its three nuclear tests in 2006, 2009 and 2013, all at the Punggye-ri site in the country's northeast.

Digging of a new test tunnel at the site which began around April 2013, paused in November 2013 and resumed in February 2014 has stopped.

But it remains unclear whether work has been completed or has just simply paused, the report said.

Last month, North Korea ramped up its nuclear threat, boasting of its ability to deliver miniaturised warheads on high-precision long range rockets.

A US National Security Council spokesman rejected the claim but agreed Pyongyang was "working on developing a number of long-range missiles, including intercontinental ballistic missiles, that could eventually threaten our allies and the homeland".