They wrote that the continued tunnelling under Mount Mantap at the Punggye-ri test site "has the potential for allowing North Korea to support additional underground nuclear tests of significantly higher explosive yields, perhaps up to 282 kilotonnes". Kilotonne is a unit of measuring the energy of explosion.
Their analysis, published on North Korea monitoring website 38 North, is based on commercial satellite imagery of the test site. The website is run by Johns Hopkins University's School of Advanced International Studies.
The report published by 38 North came a day after it warned last Thursday that North Korea could be preparing for another nuclear test.
Commercial satellite imagery taken last Tuesday of the North's Punggye-ri test site indicated movements and activities at the site's North Portal, the main administrative area and the command centre, said defence analyst Joseph Bermudez.
South Korea's Unification Ministry said North Korea is ready to conduct a nuclear test at any time.
"We are closely monitoring the situation while being fully alert against (North Korea's possible provocation)," ministry spokesman Jeong Joon Hee told a regular press briefing last week.
North Korea's state agency KCNA said the missiles were aimed at United States bases in Japan. The launches coincided with largest joint US-South Korean military exercises, yearly drills which Pyongyang views as preparation for an invasion.
In response to North Korea's latest missile launches, the US accelerated the deployment of the first elements of its advanced anti-missile defence system in South Korea despite protests from China.
Today, South Korea and the US will kick off a computer-simulated command post exercise, codenamed Key Resolve, which is aimed at improving the combined forces' operation and combat capabilities to deter threats from the North. The war game is carried out in conjunction with Foal Eagle, a field training exercise that began on March 1.
The Nimitz-class USS Carl Vinson aircraft carrier is scheduled to arrive in Busan on Wednesday or thereabouts to participate in the ongoing drills, said Yonhap news agency.
US Secretary of State Rex Tillerson is travelling to the region this week on his first trip to Asia as America's top diplomat. He will travel to Japan, South Korea and China from Wednesday to Sunday.
Long-time observers say the risk of conflict is higher than it has been in years, and it is likely to rise further as North Korean leader Kim Jong Un seeks to fulfil his pledge to field long-range missiles capable of striking US cities.
"This is no longer about a lonely dictator crying for attention or demanding negotiations," said Dr Victor Cha, the Korea chair at the Washington-based Centre for Strategic and International Studies.
"This is now a military testing programme to acquire a proven capability," he told The Washington Post.