North Korea rocket engine test
North Korean leader Kim Jong-un watched the ground jet test of a Korean-style high-thrust engine newly developed by the Academy of the National Defence Science in this undated picture KCNA via Reuters

North Korea could be in the last stages of preparation to launch a sixth nuclear test, satellite images taken at the site in Punggye-ri show.

A US based think-tank, 38 North reported on Tuesday (28 March) that the pictures from Saturday showed a constant presence of trailers and other vehicles at the Punggye-ri nuclear test site. There were also indications that communication cables could have been placed at a test tunnel.

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The vehicles might be used to deploy a nuclear device for an underground test, 38 North said.

The think-tank added that water was being pumped out to possibly keep the tunnel dry for communications and monitoring equipment.

"The combination of these factors strongly suggests that test preparations are well under way, including the installation of instrumentation. The imagery, however, does not provide any definitive evidence of either a nuclear device or the timing of a test," the think-tank said.

According to Reuters, the report also warned: "Since North Korea knows the world is watching and is capable of deception, caution should be used before declaring that a nuclear test is imminent."

The isolated country has conducted five nuclear tests so far at the Punggye-ri nuclear site, defying the United Nations sanctions.

On Friday (24 March), a South Korean military official said Pyongyang has the capability to conduct a fresh nuclear test amid rising tension in the region.

US officials confirmed on Monday that North Korea had conducted a second rocket engine test, which could be a part of its plan to develop an intercontinental ballistic missile.