A satellite handout image from DigitalGlobe dated Sep 29, 2004 shows the Yongbyon complex nuclear facility, north of Pyongyang, North Korea. EPA-EFE FILE/DIGITALGLOBE / HANDOUT MANDATORY CREDIT HANDOUT EDITORIAL USE ONLY

US President Donald J. Trump (R) speaks as North Korean leader Kim Jong-un (L) looks on during a document signing ceremony during their historic DPRK-US summit, at the Capella Hotel on Sentosa Island, Singapore, Jun 12, 2018. EPA-EFE FILE/KEVIN LIM / THE STRAITS TIMES / SPH EDITORIAL USE ONLY EDITORIAL USE ONLY

North Korea has carried out improvements at its Nuclear Scientific Research Center after signing a declaration at the Singapore summit in which it pledged to denuclearize if Washington guaranteed the regime's survival, as revealed by satellite images, analysts said Wednesday.

Satellite images captured on Jun. 21 and appraised by specialist website 38North highlight that significant infrastructure improvements on the Yongbyon Nuclear Scientific Research Center, about 80 kilometers north of Pyongyang, have continued since the historic meeting in Singapore.

United States President Donald Trump, and North Korean leader, Kim Jong-un, signed a joint declaration at the Singapore summit on Jun. 12 that paved the way for better bilateral relations and the "complete denuclearization" of the North Korean regime in exchange for Washington's guarantees of its survival.

The document does not specify any process or deadlines for meeting the objectives.

For this reason, the 38North analysts consider that continued work at the Yongbyon facility "should not be seen as having any relationship to North Korea's pledge to denuclearize."

"The North's nuclear cadre can be expected to proceed with business as usual until specific orders are issued from Pyongyang," according to 38North.

The detected improvement works include modifications to the cooling system of the plutonium production reactor, the source of the plutonium that Pyongyang has used for its nuclear tests, although it is "difficult" to determine if the unit is currently operational.

New buildings have also been constructed around the Radioisotope Production Facility, used to separate the plutonium from the used fuel rods, and at the Experimental Light Water Reactor.

Yongbyon, comprising more than 600 buildings, is the center of the North Korean nuclear program and its closure and dismantling would be essential for Pyongyang to fulfill its commitment of denuclearization.