Satellite imagery shows significant new construction at North Korea's main rocket launch site in a sign of leader Kim Jong-un's determination to pursue a space programme, a US research institute has said.

The North has been upgrading the Sohae launch site on its west coast since mid-2013 after it blasted its first rocket into space in December 2012. It says the space programme is peaceful.

The US-Korea Institute at Johns Hopkins School of Advanced International Studies, which released new research on Friday, says the North completed an expansion of its launch tower late last year to take larger rockets.

The institute says commercial satellite imagery shows that since then, the North has been working on a support building and what appears to be a movable platform to allow an assembled rocket to be shifted on rails to the launch pad.

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"The Sohae facility upgrade programme represents a significant investment of financial, material and personnel resources and is another indicator, along with its public statements, that North Korea is determined to pursue its space programme," read the research institute's analysis provided to the Associated Press.

Rocket launch in 2012

North Korea is barred under UN Security Council resolutions from launching rockets as that technology can also be used to launch ballistic missiles. 

The impoverished but nuclear-armed North sent an Unha-3 rocket into space in December 2012 from the Sohae satellite launch site - a move the international community condemned as a disguised ballistic missile test and punished with further sanctions.

Two months later, the North conducted its third - and most powerful to date - nuclear test.

Concern is rising over North Korea's weapons development. The North recently claimed it tested a new type of missile from a submarine and reiterated that it had built a nuclear warhead small enough to be mounted on a long-range missile.

Outside analysts are sceptical about both claims, but they believe the North has built a small but growing nuclear bomb arsenal and advanced its missile programme since international nuclear disarmament talks stalled in early 2009.

In Seoul on Wednesday, nuclear envoys from South Korea, the US and Japan agreed on the need to increase pressure on North Korea and urged the country to engage in serious negotiations on its nuclear weapons programme.

Source: Agencies