Activity has been detected at a North Korean long-range rocket site, suggesting Pyongyang may be pursuing the "rapid rebuilding" of the facility after the collapse of the Hanoi summit, according to analysis of satellite imagery.
Another research website suggested the rebuilding of the site may have started even before last week's meeting between North Korean leader Kim Jong Un and US President Donald Trump.
Meanwhile, the United Nations said North Korea recorded its worst harvest for more than a decade last year, and just under half of its people could face food shortages.
Last week's summit ended abruptly after North Korean leader Kim Jong Un and US President Donald Trump failed to reach an agreement on pulling back Pyongyang's nuclear programme in exchange for sanctions relief.
The renewed activity at the test site was recorded two days after the talks and may "demonstrate resolve in the face of US rejection" of the North's request for an easing of sanctions, said researchers at the Washington-based Center for Strategic and International Studies (CSIS).
"This facility had been dormant since August 2018, indicating the current activity is deliberate and purposeful," it said.
Kim had agreed to shutter the Sohae missile-testing 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 suggested workers were already dismantling an engine test stand at the facility.
But CSIS said building activity is now "evident" at the Sohae Satellite Launching Station, from where Pyongyang launched satellites in 2012 and 2016.
North Korea was later banned by the UN security council from carrying out the space launches, as some of its technology was similar to that used for intercontinental ballistic missiles.
The respected Washington-based 38 North project, another independent research website specialising in North Korea, also reported building work at the Sohae facility, based on commercial satellite imagery.
The pictures show a moving structure that had been used to carry launch vehicles to a launch pad on rails has been restored, it said.
"Two support cranes are observed at the building, the walls have been erected and a new roof added. At the engine test stand, it appears that the engine support structure is being reassembled," the 38 North reported.
In a briefing to parliamentarians this week, Seoul's spy agency said it had detected signs of work at the site.
- No deal in Hanoi -
Impoverished and isolated North Korea conducted its first successful nuclear test in 2006 followed by a string of increasingly successful ICBM launches.
In 2017, it claimed it had become a nuclear state, capable of fitting a viable nuclear weapon on an ICBM that could reach as far as the United States' eastern seaboard.
In response, the UN Security Council later banned the North's main exports -- coal and other mineral resources, fisheries and textile products -- to cut off its access to hard currency.
At their groundbreaking first summit in Singapore last year, Kim and Trump produced a vague statement on the "denuclearisation of the Korean peninsula".
There were high expectations for Kim and Trump's second meeting in Hanoi, but no agreement was reached. Despite the stalemate, both sides said they were open to further talks.
John Bolton, the US national security adviser, however said on Tuesday that the US would look at "ramping those sanctions up" if Pyongyang did not give up its nuclear weapons program.
North Korea food production 'lowest for a decade'
Meanwhile, the United Nations said North Korea recorded its worst harvest for more than a decade last year, as natural disasters combined with its lack of arable land and inefficient agriculture to hit production.
The isolated North has long struggled to feed itself and suffers chronic food shortages.
But last year's harvest was just 4.95 million tonnes, the United Nations said in its Needs and Priorities assessment for 2019, down by 500,000 tonnes.
It was "the lowest production in more than a decade", the UN's Resident Coordinator in the North Tapan Mishra said in a statement.
"This has resulted in a significant food gap."
As a result 10.9 million people in the North needed humanitarian assistance -- 600,000 more than last year -- with a potential for increased malnutrition and illness.
It is equivalent to 43 percent of the population.
But while the number of people needing help rose, the UN has had to cut its target for people to help -- from 6.0 million to 3.8 million -- as it seeks to prioritise those most in need.
Funding has fallen far short of what the UN says it needs.
Only 24 percent of last year's appeal was met, with Mishra describing it as "one of the lowest funded humanitarian plans in the world".
Several agencies had been forced to scale back their programmes and some faced closing projects, he said, appealing to donors to "not let political considerations get in the way of addressing humanitarian need".
"The human cost of our inability to respond is unmeasurable," he said, adding that sanctions had created unintended delays and challenges to humanitarian programmes, even though they are exempt under UN Security Council resolutions.
- Arduous March -
The impoverished North has been frequently condemned by the international community for decades of prioritising the military and its nuclear weapons programme over adequately providing for its people -- an imbalance some critics say the UN's aid programme encourages.
Ahead of his Hanoi summit with leader Kim Jong Un last week, US President Donald Trump repeatedly dangled the prospect of the North becoming an economic powerhouse if it gave up its arsenal, but the two were unable to reach a deal.
The country industrialised rapidly following the end of the Korean War and for a time was wealthier than the South. Funding from Moscow papered over the effects of chronic economic mismanagement, but that came to an end with the collapse of the Soviet Union, which was followed by a crippling famine.
That episode -- known as the Arduous March, when hundreds of thousands of people died -- is in the past but North Korea does not have access to the latest agricultural technology or fertilisers and its yields are well below global averages.
It is also a rugged, largely mountainous, country with only around 20 percent of its land area suitable for cultivation.
It was hit by a heatwave in July and August last year, followed by heavy rains and flash floods from Typhoon Soulik. As a result, the UN said, rice and wheat crops were down 12 to 14 percent.
The figure is significantly larger than in the South, where rice production was down only 2.6 percent last year, according to Seoul's statistics, even though it experiences similar weather and climate.
The North's soybean output slumped 39 percent and production of potatoes -- promoted by leader Kim as a way to increase supplies -- was 34 percent down, the UN said.
Last month Pyongyang told the UN that it was facing a shortfall of 1.4 million tonnes of food this year.
Mishra insisted that humanitarian agencies were able to monitor their programmes "rigorously" to make sure that help reached the most vulnerable people, adding: "We simply cannot leave them behind."