A report on 38 North, a website run by the US-Korea Institute at the Johns Hopkins School of Advanced International Studies, studied images taken between April and November.
These appeared to show work in a new area of the Punggye-ri nuclear zone.
But, the report said, there is no sign that any nuclear test is imminent,
North Korea carried out three underground nuclear tests at Punggye-ri in 2006, 2009 and 2013.  Its nuclear programme has been a source of great concern for the international community.
The tunnel identified in the latest imagery is in a new area of the site, separate from three other tunnels that the North has excavated or used for tests in the past, the report said.
"While there are no indications that a nuclear test is imminent, the new tunnel adds to North Korea's ability to conduct additional detonations over the coming years if it chooses to do so," it added.
The commercially-available satellite imagery on which the researchers based their conclusions appear to show a new tunnel entrance, the site's fourth, as well as signs of construction work taking place as recently as October and November.
The report says some analysts believe each entrance connects to a single tunnel but there is debate about the exact nature and structure of the underground testing facility.
It is not the first indication of construction activity at the site.  In October, South Korea's Yonhap news agency quoted a South Korean government source saying there were workers constructing a new tunnel at Punggye-ri.
In September North Korea said that its reactor at the Yongbyon nuclear facility, the source of plutonium for North Korea's nuclear weapons, had resumed normal operations.
Yongbyon's reactor was shut down in 2007 but Pyongyang vowed to restart it in 2013, following its third nuclear test and amid high regional tensions.

After hosting the British Science Festival last year the university wants to capitalise on its success.
Visitors will be able to see demonstrations and workshops based on everyday science through to the latest cutting-edge innovations.
"We continue to inspire the next generation of scientists," said Professor Hilary Lappin-Scott.
The university's senior pro-vice-chancellor added hosting the British Science Festival was an "enormous success" and she wants it to create a "science legacy for Swansea".
It brought tens of thousands of people to the city to celebrate developments and talk about issues affecting our culture and society.
The new festival will be held at the city's National Waterfront Museum from 8-10 September.

A report by NHS Shetland has suggested the move, in a bid to curb excessive drinking.
About three-quarters of all alcohol consumed in Scotland is now bought from supermarkets or off licences.
The proposal will be considered by the local licensing board after the council elections next year.
Elizabeth Robinson, one of the report authors from NHS Shetland, told BBC Scotland: "People with alcohol problems have told us that they can go out the next morning and buy alcohol at 10am.
"If there was just a bit of a longer break until some time in the afternoon that would really help them.
"You can buy your 14 units a week for about Â£3 in Shetland.
"People tend to pour larger drinks at home."
However, shopkeeper Jordan Thomason said: "I think such a drastic increase in time runs the risk of people buying too much alcohol and then it could have the opposite effect of what they are trying to do."
Recent satellite images indicate that North Korea is building a new tunnel at its nuclear test site, a respected US think tank has said.
[NEXT_CONCEPT]
Swansea University will celebrate its achievements and research by holding a new science festival in September.
[NEXT_CONCEPT]
A proposal that supermarkets and off licences should only be able to sell alcohol later in the day is to be considered in Shetland.