WASHINGTON — Satellite imagery shows North Korea is upgrading its old launch site in the secretive country’s northeast to handle larger rockets, such as space launch vehicles and intercontinental missiles, a U.S. institute said Tuesday.

The U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies said the upgrade of the Musudan-ri site began last summer and reflects North Korean determination to expand its rocket program.

The U.S. and other nations are worried such rockets could be developed to deliver nuclear weapons.

North Korea on Tuesday vowed to push ahead with its nuclear program because of what it called U.S. hostility.

An April 29 aerial image of Musudan-ri shows the initial stages of construction of a launch pad and rocket assembly building that could support rockets at least as big as the Unha-3, North Korea’s biggest rocket to date and one that failed to launch during an attempt in mid-April, the institute told The Associated Press.