Fidelity International is calling for urgent action to address the humanitarian crisis and supply-chain risks caused by the more than 400,000 seafarers who are stranded aboard vessels and a similar number who remain ashore with little prospect for work or pay.

The London-based money manager, which oversees more than $600 billion, plans to write to the United Nations next week and has asked other investors to co-sign the letter. Fidelity has so far stood out among fund companies in highlighting the plight of seafarers.

“As investors, it is clear that this is no longer solely a shipping-industry problem nor a crisis that the shipping industry can resolve on their own,” Fidelity International wrote in a note posted on the website of the Principles for Responsible Investment, which is a global forum for investment firms.

A Bloomberg investigation published in September found numerous violations of international maritime law designed to protect seafarers, including allegations of unpaid overtime and insufficient medical attention. More than 120 countries or territories had stopped or limited access for ships to conduct seafarer changes in a bid to prevent the spread of the coronavirus. And while many shipping lines had managed piecemeal crew changes, the backlog of crew swaps has far outpaced relief efforts.

The UN General Assembly adopted a resolution earlier this month calling on member states to designate seafarers and other marine personnel as key workers and to take steps to help stranded sailors be repatriated. The world’s largest shipping companies, including A.P. Moller-Maersk A/S, also have appealed to governments for help to resolve the crisis.

Earlier this year, Fidelity International, which is independent from Boston-based Fidelity Investments, wrote to more than 30 companies in the shipping and charter sectors asking them to address the problem. The issue causes health and safety risks to sea workers and may lead to environmental damage if exhausted seafarers have a maritime accident.

Fidelity International added Wednesday in its statement that companies will come under increasing pressure to take responsibility for the welfare of their employees, as well as people working in their supply chains. The issue will be a key priority in 2021 for Fidelity International’s engagement team, which is tasked with lobbying companies to make changes. The money manager said it also will focus on biodiversity loss and “digital ethics.”