North Korea appears to be expanding its main launch site to permit more advanced missiles which may eventually be able to reach the United States, a think tank said Wednesday.

Analyzing satellite images of the Sohae launch site over the past two months, Johns Hopkins University's US-Korea Institute said North Korea apparently tested a rocket engine needed for its road-mobile KN-08 intercontinental ballistic missile.

The evidence indicates that North Korea may be preparing "for a more robust rocket test program in the future," said the institute's blog, 38 North.

This expansion could involve "larger space launch vehicles and road-mobile ballistic missiles able to attack targets in Northeast Asia and the United States."

Researchers have repeatedly said North Korea is expanding its nuclear weapons and missile programs, amid questions about the regime's internal stability after young leader Kim Jong-Un executed his uncle and former mentor.

The 38 North blog said North Korea appears to be modifying the Sohae site to allow rockets up to 25 percent longer than the Unha-3, which successfully put a small satellite into orbit in December 2012.

The construction means that North Korea is unlikely to be able to carry out any new launches until March or April, it said.

In January 2011, then defense secretary Robert Gates said that North Korea could develop intercontinental missiles capable of hitting the US Pacific coast within five years.

Diplomacy has been at a standstill, with the United States insisting that North Korea show a willingness to wind down its nuclear weapons program before any talks.