Satellite imagery shows North Korea is upgrading its old launch site in the secretive country's northeast to handle larger rockets, like space launch vehicles and intercontinental missiles, a U.S. institute claimed on Monday.

The U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies says 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.

The North used a different, more sophisticated site at Sohae on the country's northwestern coast for a failed attempt in mid-April to launch a satellite using its biggest rocket to date, the Unha-3.

The institute says that 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. A crane is visible where the launch pad is being built 1.1 miles from the old one. At the current pace of construction, the facilities should be operational by 2016-2017, the institute says.

"This major upgrade program, designed to enable Musudan-ri to launch bigger and better rockets far into the future, represents both a significant resource commitment and an important sign of North Korea's determination," said Joel Wit, editor of the institute's website, 38 North.

The institute says the assembly building shows similarities to one at the Seman launch complex in Iran, which has a long history of missile cooperation with North Korea. But, officials there say it's premature to conclude the two nations cooperated in designing the new facility.

The revelation of the work at Musudan-ri comes as the international community pressures North Korea to refrain from conducting a third nuclear test. Both of its previous nuclear tests, in 2006 and 2009, followed rocket launches.

The top U.S. envoy on North Korea, Glyn Davies, who is meeting this week with counterparts from Japan, South Korea and China, warned Monday that the North conducting an atomic test would unify the world in seeking swift, tough punishment.

The April rocket launch drew U.N. Security Council condemnation, as the launch violated an existing ban. Similar technology is used for ballistic missiles. The North, however, is not believed to have mastered how to wed a nuclear device to a missile.

The upgrade at Musudan-ri could be of particular concern to Japan, as rockets launched from the site in the past have flown east over that country. The flight path from Sohae heads south over the Pacific Ocean in the direction of Southeast Asia, avoiding Japan and South Korea.

Land-clearing at the new site began in the fall, and work has proceeded at a fast pace for eight months, the institute says, citing previous satellite imagery.

The latest image, from a commercially operated satellite, shows four concrete footings on one side of the launch pad that appear to be for a gantry that would prop up a rocket at launch. It has bigger dimensions than the gantry at Sohae.

On another side of the launch pad there is a deep ‘flame trench’ to capture the blast from a launched rocket. Slightly further away, on either side of the launch pad, are two separate buildings designed to enclose the fuel and oxidizer tanks that would funnel propellant into the rocket.

Satellite imagery also shows that about 70 homes, 5 larger buildings and many sheds in the nearby village of Taepodong have been razed and foundations laid for a large T-shaped structure that appears intended for assembling rockets. A road is under construction that would lead from this building to the launch site, 1.2 miles away.

The building's dimensions are larger than at the comparable structure at Sohae, and the existing one at Musudan-ri, the institute says.