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 program despite international censure, a U.S. research institute said Thursday.

North Korea is barred under U.N. Security Council resolutions from launching rockets as that technology can also be used to launch ballistic missiles. Kim, however, declared this month that its space program "can never be abandoned."

North Korea 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 program is peaceful.

The U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies says the North completed an expansion of its launch tower late last year to take larger rockets. Commercial satellite imagery shows that since then, the North has been working on a support building and what appears to be a moveable platform to allow an assembled rocket to be shifted on rails to the launch pad. The institute likens it to facilities in China but says there's no publicly available evidence to suggest Beijing is providing help.

The most recent image is from May 16.

"The Sohae facility upgrade program 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 program," read the analysis provided to The Associated Press ahead of publication on the institute's website, 38 North.

Satellite imagery analyst Tim Brown writes that the expansion of the launch tower suggests the North wants to field a larger space launch vehicle, which may also contribute to its development of long-range ballistic missiles.

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

In Seoul on Wednesday, nuclear envoys from South Korea, the United States 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 program.

The North's unpredictable leader, Kim, has closely associated himself with the space program. In early May, state media reported Kim inspected a newly-built satellite control and command center, but didn't specify the location. Kim was quoted as saying the North would launch satellites into space at the time and locations chosen by the ruling party.

The December 2012 launch propelled a satellite into space, but U.S. officials said it soon tumbled out of control in orbit.

There is speculation that North Korea could mark the 70th anniversary this October of the ruling Korean Workers Party with a rocket launch. The institute says it would be difficult to complete construction at the launch site by then, but the North could go ahead with a launch by pausing construction and using the existing facilities at Sohae.