Analysis published Sunday of recent satellite images fuelled concerns that North Korea may be on the brink of another nuclear test or long-range rocket launch.

Speculation that Pyongyang is preparing such a show of force has been linked to Monday's anniversary of the founding of the North's ruling Workers' Party.

Past nuclear tests and missile launches have often coincided with key political dates. Its fifth nuclear test last month was conducted on the anniversary of North Korea's founding as a state,

The latest satellite imagery analysis posted by the US-Korea Institute at Johns Hopkins University noted increased activity at the North's Sohae satellite launch station.

The pictures taken on October 1 showed crates on the launch pad next to the gantry tower, vehicles near the fuel and oxidiser buildings, and work continuing on the facility's vertical engine test stand.

"However, since both the gantry tower and the assembly structures on the launch pad are covered, it is unclear whether this activity is related to launch preparations or other operations," the analysts noted.

Two days ago, the institute had posted similarly dated images of the North's nuclear test site that showed activity at all three of its tunnel complexes.

But again its analysts could not be certain if the activity was related to an imminent test or other work.

Sunday marked 10 years to the day that North Korea carried out its first nuclear test on October 9, 2006 -- an underground detonation with such a low yield that it was widely seen as a failure.

But the North's weapons programme has progressed in leaps and bounds since then -- despite rounds of increasingly tough international sanctions -- and has notably accelerated under current leader Kim Jong-Un.

Since taking power following the death of his father Kim Jong-Il in late 2011, Kim has overseen three nuclear tests -- two of them in this year alone.

Each has shown a significant level of progression, with September's fifth test the largest to date, and Pyongyang also claims it has mastered the miniaturisation technique to fit a nuclear warhead on the tip of a missile.

The final goal of the North's programme is a credible nuclear strike capability against the US mainland.

While most experts don't believe it is there yet, they generally agree that the level of bomb and missile testing -- especially over the past year -- has brought it much closer.

The North carried out its last successful satellite rocket launch in February -- a month after its fourth nuclear test.

Pyongyang insists such launches are purely scientific, but the international community has condemned them as disguised ballistic missile tests.

Last month, North Korea successfully tested a new, high-powered rocket engine, a move Seoul said was designed to showcase its progress towards being able to target the US east coast.

And in August it carried out its most successful test to date of a submarine-launched ballistic missile that would allow deployment far beyond the Korean peninsula.