"Given that construction, plus activity at other areas of the site, Sohae (Satellite Launching Station) appears to have returned to normal operational status," the specialized website 38 North said.

The website and the Center for Strategic and International Studies tracked activity at the site -- which began before last week's aborted summit in Hanoi between US President Donald Trump and North Korea's Kim Jong Un -- using commercial satellite imagery.

Images taken on March 6 showed that a rail-mounted structure to transfer rockets to the launching pad appeared to have been completed and "may now be operational."

Cranes have been removed from pad while progress also appeared to have been made on rebuilding the support structure for a rocket engine testing stand.

Kim had agreed to shutter the Sohae site at a summit with the South's President Moon Jae-in in Pyongyang as part of confidence-building measures, and satellite pictures in August had suggested workers were already dismantling the engine test stand.

Pyongyang used the site in 2012 and 2016 to launch satellites.

Western experts believe the satellite launches inform North Korea's development of inter-continental missiles capable of striking the United States.

CIA director Gina Haspel said in late January that North Korea remains committed to developing long-range missiles despite its denuclearization talks with the US.

Asked Wednesday about the renewed activity at the site, Trump said it was "too early" to tell if the reports were true, but if confirmed, he would be "very, very disappointed in Chairman Kim."

Trump and Kim abruptly ended a summit in Hanoi on February 28 without an agreement or even a joint statement.

"Sometimes you have to walk," Trump said at a news conference afterwards.