Satellite imagery indicates that North Korea has completed upgrades to a rocket launchpad in the country's northwest and is proceeding with work to install a cover over it, diplomatic sources familiar with the developments said Saturday.

U.S. intelligence agencies, believe the cover at the launch site in Tongchang-ri, North Pyongan province, is an attempt to evade surveillance by spy satellites, the sources said.

There are no indications that a rocket has been placed on the pad at the site, called the Sohae Satellite Launching Station.