WASHINGTON (AP) — A U.S. research institute says North Korea is upgrading its main rocket launch site and has conducted a series of engine tests as it develops a mobile, intercontinental missile that could threaten the U.S.

The findings are based on satellite photos of the west coast site of Sohae, analyzed by the U.S.-Korea Institute.

North Korea blasted a rocket into space from site a year-and-half ago, and the images indicate it has made significant, recent progress upgrading facilities at Sohae to handle bigger rockets.

The flurry of activity comes as North Korea has been test-firing short-range missiles, drawing U.N. Security Council condemnation this month.

In its analysis Tuesday, the institute also detects the North conducted its fourth test this year of an engine that could be used for the mobile missile.