NEW YORK (NYTIMES) - President Donald Trump has called the coronavirus "the invisible enemy." But when it comes to sanctions on North Korea, the pathogen may turn out to be his administration's most effective ally.

North Korea's fear of coronavirus infection appears to have achieved what Mr Trump's "maximum pressure" campaign against North Korean nuclear and missile work has not: choking the North's economy by stopping its trafficking of coal and other goods, prohibited under United Nations sanctions, which is believed to be mostly with China.

According to a satellite-image analysis published last Thursday (March 26) by the Royal United Services Institute, a London-based research organisation, and a review of additional satellite imagery by The New York Times, many North Korean commercial vessels that once carried sanctioned material to and from China - or transferred them illicitly ship-to-ship at sea - are now idled in their home ports.

The change, seen after North Korea sought to seal itself off two months ago as neighbouring China battled the coronavirus outbreak, effectively puts "an end to the large and coordinated efforts to evade United Nations Security Council Resolutions (UNSC) by shipping resources to China," according to the analysis.

The State Department declined through a spokesman to comment on the analysis or its implications.

The new findings show for the first time how a large number of North Korean ships have moved back to Nampo, a vital port region on North Korea's western coast.

UN sanctions ban the export of many North Korean commodities - such as coal - and limit its fuel imports. However, a draft of the forthcoming annual report from a UN panel of experts that monitors compliance with the sanctions, which was seen by The Times, states that North Korea "continued to flout UN Security Council resolutions through illicit maritime exports of commodities, notably coal and sand.

Such sales provide a revenue stream that has historically contributed to its nuclear and ballistic missile programmes. The UN report only covers the time frame up to early February, and does not account for the impact of the coronavirus pandemic.

The Royal United Services Institute satellite analysis shows that on March 3, 139 ships were idled in the Nampo area, which includes the anchorage and several ports, up from 50 ships a month earlier.

The fleet includes vessels previously implicated in sanctions evasion operations, which are often tracked through satellite imagery and aerial or ground surveillance by other states, independent research groups and the UN.

The institute's analysis said the idled ships included some of the "most active and scrutinised oil tankers" used for the illicit import of refined petroleum products such as fuel. For example, the oil tanker New Regent, which had been spotted making unreported deliveries as recently as January, and twice in 2019, according to the UN, was seen in Nampo in multiple satellite images.

Other ships, too, have been floating unused for weeks, according to satellite imagery provided by Planet Labs, an earth-imaging company in San Francisco, and Maxar Technologies Inc, a space technology company in Westminster, Colorado.

The possible drop in fuel imports, which are often done through illicit ship-to-ship transfers and deliveries involving vessels such as the New Regent, would affect North Korea's agriculture sector, right at the beginning of the farming season. Diesel is used for powering water pumps in agriculture.

And the overall current restrictions on legitimate trade, also attributable to the pandemic, carry additional risks for North Korean farming.

"There could be disruptions of the food planting season, which starts now, for example, because of an interruption of fertiliser imports from China," said Mr Daniel Wertz, programme manager for the National Committee on North Korea, a Washington-based group that advocates improved ties between North Korea and the US.

An increase in the number of idled North Korean cargo ships can also be seen in Chongjin, another important industrial port city on the country's east coast, 80km from the Chinese border.

The recall of ships is part of North Korea's border closures, which started on Jan 22, the same time the Chinese authorities announced the shutdown of the city of Wuhan, the epicentre of the coronavirus outbreak.

Prof Park Jong-chol, professor at the Department of Social Studies Education at South Korea's Gyeongsang National University, said the economic impact on North Korea from idling so many ships is severe.

"Official and informal trade is now suspended. Therefore, transportation systems such as ships and trucks have been drastically reduced," he said.

"Corona is more threatening than UN Security Council sanctions," he said. "Production and income, including gasoline, have been an issue, and many ships have been unable to operate."

One returned ship, the Tian Tong, suspected of transporting North Korean coal to China in October 2019, had been spotted near China's Zhoushan Island, about 960km south of Nampo, on Jan 31 using a falsified transponder signal to disguise its North Korean origin, a tactic used to conduct illicit trade. It returned to North Korea in early February, and has been anchored outside Nampo since at least Feb 14, according to ship tracking data and satellite imagery reviewed by The Times.

The long-term disruptions to North Korea's revenue stream remain unclear, partly because the duration of the pandemic and its impact on maritime commerce are not yet known.

But analysts said it was reasonable to assume damage has been done to North Korean agriculture, industry and the overall economy.

"If their exports are declining, and coal smuggling or other operations are interrupted, that might definitely have an impact," Mr Wertz said.

In an analysis of China-North Korea trade published last month by 38North, a website that specialises in North Korea, Mr Wertz wrote ominously about the potential impact of the coronavirus: "The next few months may provide a grim natural experiment showing what might happen to the North Korean economy if its economic linkages with China truly begin to be severed."